Fix coding of maximum and minimum time-stats in Grib2 save.

These were swapped around.

grib spec has:
```
Code table 4.10 – Type of statistical processing
Code figure Meaning
0 Average
1 Accumulation
2 Maximum
3 Minimum
4 Difference (value at the end of time range minus value at the beginning)
...
```

But existing code has min+max swapped around.