Ensure all variable attributes comply with CF conventions

[xaviernogueira]

We want to align our variable attributes (i.e., names, long names, units, and descriptions) with CF conventions: http://cfconventions.org/

I quickly ran thru and added variables to get the whole refactor working and demo-able, and did NOT check if they were valid!

@aufdenkampe feel free to assign this as you see fit

[aufdenkampe]

We might want to leverage the cf_xarray library to help us develop and confirm the Climate and Forecast metadata convention attributes that we assign to variables that become xarray objects.

cf_xarray also includes the cf_xarray.units module that can work with pint and pint-xarray to create unit-aware arrays that work well with UDUNITS format recommended by the CF conventions.

The cfunits is another package that could potentially be useful.

[sjordan29]

Per @aufdenkampe's recommendation as part of PR #78, but when we do this, "I would like to maintain some type of attribution to the old name, which was inherited from the Fortran code. I'm not sure the best way to do that. Maybe append to the description "Formerly 'TwaterK'"?"

[aufdenkampe]

@sjordan29, an alternative approach to retaining some link to the old name might be to add a new variable attribute, such as old_name or alias.