Alpine-3D: Write grids to variable specific NetCDF file

[EricKeenan]

Currently, all specified grids are written to a single NetCDF file. For my purposes, these files are becoming too large. For me, it would be preferred to write individual files for each individual year. We could also consider adding an experiment id with information about forcing, spinup, etc.

e.g.

WAIS_MERRA2_100m_spinup_T2M_1980_hourly.nc 
WAIS_MERRA2_100m_spinup_SWE_1980_hourly.nc
....
....
WAIS_MERRA2_100m_spinup_T2M_2020_hourly.nc 
WAIS_MERRA2_100m_spinup_SWE_2020_hourly.nc

[nwever]

Commit a0d010d adds the option to split output NetCDF files by using the keys:
NETCDF_SPLIT_BY_YEAR = TRUE
NETCDF_SPLIT_BY_VAR = TRUE

Labelling the output file can be steered for now by the key GRID2DFILE.

[EricKeenan]

Thanks! The NETCDF_SPLIT_BY_YEAR = TRUE feature appears to work great. I have not yet tested NETCDF_SPLIT_BY_VAR = TRUE, but when I do I will report here.