Element finder

Element finder is used to find all possible element lines close to a given energy value.
This should be 
```
true_name = sorted(['Eu', 'Cu'])
out = emission_line_search(8, 0.05, 10)
found_name = sorted(list(six.iterkeys(out)))
assert_equal(true_name, found_name)
```

You should not be testing string equality with arrays.

You need to sort the returned keys as they can come out in any order.

Another option is to turn them into a set.