Interpolate the density in addition to the temperature and salinity (to decrease discrepancies between the FVCOM data and the POLCOMS data)
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).
There is a very important but subtle difference between "less than" and "less than or equal to" (ditto "more than"/"more than or equal to"). The net result was that valid minima were being replaced with NaNs, which caused NaNs to get written to the FVCOM input file which in turn causes the model to crash at the first time step