Add missing license in `NumpyFormatter`

## What's in this PR?

As discussed with @lhoestq in https://github.com/huggingface/datasets/pull/5522, the license for `NumpyFormatter` at `datasets/formatting/np_formatter.py` was missing, but present on the rest of the `formatting/*.py` files. So this PR is basically to include it there.

_The documentation is not available anymore as the PR was closed or merged._

<details>
<summary>Show benchmarks</summary>

PyArrow==6.0.0

<details>
<summary>Show updated benchmarks!</summary>

### Benchmark: benchmark_array_xd.json

| metric | read_batch_formatted_as_numpy after write_array2d | read_batch_formatted_as_numpy after write_flattened_sequence | read_batch_formatted_as_numpy after write_nested_sequence | read_batch_unformated after write_array2d | read_batch_unformated after write_flattened_sequence | read_batch_unformated after write_nested_sequence | read_col_formatted_as_numpy after write_array2d | read_col_formatted_as_numpy after write_flattened_sequence | read_col_formatted_as_numpy after write_nested_sequence | read_col_unformated after write_array2d | read_col_unformated after write_flattened_sequence | read_col_unformated after write_nested_sequence | read_formatted_as_numpy after write_array2d | read_formatted_as_numpy after write_flattened_sequence | read_formatted_as_numpy after write_nested_sequence | read_unformated after write_array2d | read_unformated after write_flattened_sequence | read_unformated after write_nested_sequence | write_array2d | write_flattened_sequence | write_nested_sequence |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.008837 / 0.011353 (-0.002516) | 0.004608 / 0.011008 (-0.006400) | 0.101821 / 0.038508 (0.063312) | 0.030300 / 0.023109 (0.007191) | 0.301275 / 0.275898 (0.025377) | 0.365027 / 0.323480 (0.041547) | 0.007043 / 0.007986 (-0.000943) | 0.003493 / 0.004328 (-0.000835) | 0.078444 / 0.004250 (0.074194) | 0.036963 / 0.037052 (-0.000089) | 0.310510 / 0.258489 (0.052020) | 0.343769 / 0.293841 (0.049928) | 0.033560 / 0.128546 (-0.094986) | 0.011427 / 0.075646 (-0.064220) | 0.323542 / 0.419271 (-0.095730) | 0.043063 / 0.043533 (-0.000470) | 0.308869 / 0.255139 (0.053730) | 0.326436 / 0.283200 (0.043236) | 0.091775 / 0.141683 (-0.049908) | 1.471020 / 1.452155 (0.018865) | 1.494328 / 1.492716 (0.001612) |

### Benchmark: benchmark_getitem\_100B.json

| metric | get_batch_of\_1024\_random_rows | get_batch_of\_1024\_rows | get_first_row | get_last_row |
|--------|---|---|---|---|
| new / old (diff) | 0.009299 / 0.018006 (-0.008707) | 0.415705 / 0.000490 (0.415215) | 0.002406 / 0.000200 (0.002206) | 0.000066 / 0.000054 (0.000012) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.022959 / 0.037411 (-0.014452) | 0.097111 / 0.014526 (0.082585) | 0.103399 / 0.176557 (-0.073157) | 0.144385 / 0.737135 (-0.592750) | 0.109069 / 0.296338 (-0.187269) |

### Benchmark: benchmark_iterating.json

| metric | read 5000 | read 50000 | read_batch 50000 10 | read_batch 50000 100 | read_batch 50000 1000 | read_formatted numpy 5000 | read_formatted pandas 5000 | read_formatted tensorflow 5000 | read_formatted torch 5000 | read_formatted_batch numpy 5000 10 | read_formatted_batch numpy 5000 1000 | shuffled read 5000 | shuffled read 50000 | shuffled read_batch 50000 10 | shuffled read_batch 50000 100 | shuffled read_batch 50000 1000 | shuffled read_formatted numpy 5000 | shuffled read_formatted_batch numpy 5000 10 | shuffled read_formatted_batch numpy 5000 1000 |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.417796 / 0.215209 (0.202587) | 4.158198 / 2.077655 (2.080543) | 1.862036 / 1.504120 (0.357916) | 1.650130 / 1.541195 (0.108936) | 1.717150 / 1.468490 (0.248660) | 0.691704 / 4.584777 (-3.893073) | 3.328254 / 3.745712 (-0.417458) | 1.850070 / 5.269862 (-3.419792) | 1.154331 / 4.565676 (-3.411346) | 0.082199 / 0.424275 (-0.342076) | 0.012226 / 0.007607 (0.004619) | 0.522491 / 0.226044 (0.296446) | 5.244181 / 2.268929 (2.975253) | 2.286651 / 55.444624 (-53.157973) | 1.954439 / 6.876477 (-4.922038) | 1.992052 / 2.142072 (-0.150020) | 0.804779 / 4.805227 (-4.000449) | 0.147341 / 6.500664 (-6.353323) | 0.063863 / 0.075469 (-0.011606) |

### Benchmark: benchmark_map_filter.json

| metric | filter | map fast-tokenizer batched | map identity | map identity batched | map no-op batched | map no-op batched numpy | map no-op batched pandas | map no-op batched pytorch | map no-op batched tensorflow |
|--------|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 1.270778 / 1.841788 (-0.571010) | 13.676378 / 8.074308 (5.602070) | 14.253498 / 10.191392 (4.062106) | 0.170748 / 0.680424 (-0.509676) | 0.028451 / 0.534201 (-0.505750) | 0.395034 / 0.579283 (-0.184249) | 0.407512 / 0.434364 (-0.026852) | 0.466740 / 0.540337 (-0.073598) | 0.564338 / 1.386936 (-0.822598) |

</details>
PyArrow==latest

<details>
<summary>Show updated benchmarks!</summary>

### Benchmark: benchmark_array_xd.json

| metric | read_batch_formatted_as_numpy after write_array2d | read_batch_formatted_as_numpy after write_flattened_sequence | read_batch_formatted_as_numpy after write_nested_sequence | read_batch_unformated after write_array2d | read_batch_unformated after write_flattened_sequence | read_batch_unformated after write_nested_sequence | read_col_formatted_as_numpy after write_array2d | read_col_formatted_as_numpy after write_flattened_sequence | read_col_formatted_as_numpy after write_nested_sequence | read_col_unformated after write_array2d | read_col_unformated after write_flattened_sequence | read_col_unformated after write_nested_sequence | read_formatted_as_numpy after write_array2d | read_formatted_as_numpy after write_flattened_sequence | read_formatted_as_numpy after write_nested_sequence | read_unformated after write_array2d | read_unformated after write_flattened_sequence | read_unformated after write_nested_sequence | write_array2d | write_flattened_sequence | write_nested_sequence |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.006733 / 0.011353 (-0.004620) | 0.004635 / 0.011008 (-0.006373) | 0.075464 / 0.038508 (0.036956) | 0.027732 / 0.023109 (0.004623) | 0.343622 / 0.275898 (0.067724) | 0.380388 / 0.323480 (0.056908) | 0.005177 / 0.007986 (-0.002808) | 0.003435 / 0.004328 (-0.000893) | 0.074546 / 0.004250 (0.070296) | 0.039115 / 0.037052 (0.002063) | 0.342207 / 0.258489 (0.083718) | 0.390324 / 0.293841 (0.096483) | 0.031665 / 0.128546 (-0.096882) | 0.011695 / 0.075646 (-0.063951) | 0.085788 / 0.419271 (-0.333484) | 0.042423 / 0.043533 (-0.001110) | 0.340748 / 0.255139 (0.085609) | 0.372813 / 0.283200 (0.089614) | 0.092395 / 0.141683 (-0.049288) | 1.502158 / 1.452155 (0.050004) | 1.618233 / 1.492716 (0.125516) |

### Benchmark: benchmark_getitem\_100B.json

| metric | get_batch_of\_1024\_random_rows | get_batch_of\_1024\_rows | get_first_row | get_last_row |
|--------|---|---|---|---|
| new / old (diff) | 0.224451 / 0.018006 (0.206444) | 0.398712 / 0.000490 (0.398222) | 0.002739 / 0.000200 (0.002539) | 0.000074 / 0.000054 (0.000020) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.025393 / 0.037411 (-0.012018) | 0.100480 / 0.014526 (0.085954) | 0.106913 / 0.176557 (-0.069644) | 0.148639 / 0.737135 (-0.588496) | 0.110098 / 0.296338 (-0.186240) |

### Benchmark: benchmark_iterating.json

| metric | read 5000 | read 50000 | read_batch 50000 10 | read_batch 50000 100 | read_batch 50000 1000 | read_formatted numpy 5000 | read_formatted pandas 5000 | read_formatted tensorflow 5000 | read_formatted torch 5000 | read_formatted_batch numpy 5000 10 | read_formatted_batch numpy 5000 1000 | shuffled read 5000 | shuffled read 50000 | shuffled read_batch 50000 10 | shuffled read_batch 50000 100 | shuffled read_batch 50000 1000 | shuffled read_formatted numpy 5000 | shuffled read_formatted_batch numpy 5000 10 | shuffled read_formatted_batch numpy 5000 1000 |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.439359 / 0.215209 (0.224150) | 4.396801 / 2.077655 (2.319146) | 2.069809 / 1.504120 (0.565689) | 1.851014 / 1.541195 (0.309820) | 1.885003 / 1.468490 (0.416513) | 0.701387 / 4.584777 (-3.883390) | 3.404943 / 3.745712 (-0.340769) | 1.874506 / 5.269862 (-3.395355) | 1.174925 / 4.565676 (-3.390752) | 0.083282 / 0.424275 (-0.340993) | 0.012352 / 0.007607 (0.004745) | 0.543058 / 0.226044 (0.317013) | 5.458186 / 2.268929 (3.189258) | 2.562159 / 55.444624 (-52.882466) | 2.198810 / 6.876477 (-4.677667) | 2.238976 / 2.142072 (0.096903) | 0.810958 / 4.805227 (-3.994269) | 0.153341 / 6.500664 (-6.347323) | 0.067773 / 0.075469 (-0.007696) |

### Benchmark: benchmark_map_filter.json

| metric | filter | map fast-tokenizer batched | map identity | map identity batched | map no-op batched | map no-op batched numpy | map no-op batched pandas | map no-op batched pytorch | map no-op batched tensorflow |
|--------|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 1.303938 / 1.841788 (-0.537850) | 14.170363 / 8.074308 (6.096055) | 13.727012 / 10.191392 (3.535620) | 0.129118 / 0.680424 (-0.551306) | 0.016746 / 0.534201 (-0.517455) | 0.382759 / 0.579283 (-0.196524) | 0.391070 / 0.434364 (-0.043294) | 0.461197 / 0.540337 (-0.079141) | 0.557641 / 1.386936 (-0.829295) |

</details>
</details>

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