Three main changes:

1. Remove the pevpr data from the list of data to be retrieved (since
   it's not used in the calculations and it is only land-based anyway).
2. Change the sign of the evaporation data (Et) to match the
   precipitation (FVCOM wants preciptation and evaporation to both have
   the same signs - ocean losing water is negative).
3. Replace data outside the actual range with NaNs. This might cause
   some things to break later on (e.g. interpolation).