Custom DOMAIN for fetchnwp

[williamhobbs]

To significantly reduce NWP file sizes, I've been modifying the DOMAIN in /solarforecastarbiter/io/nwp.py, e.g., for a region in the Southeast:

DOMAIN = {'subregion': '',
          'leftlon': -90.5,
          'rightlon': -80.5,
          'toplat': 35.2,
          'bottomlat': 30}

Maybe there's a better way to do this, but if not, would it make sense to allow a custom domain to be defined through the command line interface as an option for fetchnwp?

Similar, but maybe less useful, would be the ability to define an upper limit on the valid hour for a model. For example, if someone wants to use the GEFS for a probabilistic forecast, but they are not interested in forecasting beyond 24 hours, they could avoid having to save data for hours 30-384 (or whatever the relevant intervals are for GEFS).

[wholmgren]

From a quick glance, it seems reasonable to add those arguments to cli.fetchnwp and propagate them down through io.fetch.nwp.run and helper functions.

Considerations:

• I'm not sure if we'd want to specify the domain as a single complicated argument or as 4 or 5 different arguments on the command line
• GFS/GEFS need to be pulled in multiples of 6 hours due to how the data is structured.
• reference_observations.py uses the default DOMAIN as a check when creating reference forecasts, but I think that potential inconsistency can be ignored.

I suppose another option is reading from environment variables e.g.

DOMAIN = {'subregion': '',
          'leftlon': os.environ.get('SFA_DOMAIN_LEFTLON', -90.5),
          'rightlon': -80.5,  # and so on...
          'toplat': 35.2,
          'bottomlat': 30}