Add precipitation phase information from NLDAS-2 and MERRA-2

[SnowHydrology]

In addition to the GPM IMERG PLP values, we can also grab precipitation phase information from NLDAS-2 and MERRA-2 for each citizen science observation.

[nayhur]

I've actually started on this task - done with MERRA2, working on NLDAS2!