refactor ssp_anomaly_forcing

[samsrabin]

Description of changes

Refactors ssp_anomaly_forcing script to make it easier to read and more amenable to future development.

I originally started this in an attempt to resolve this script "stalling out" for me, but was unsuccessful—see #2181, which I'm now closing. I didn't want that work to go to waste, though, so I figured I would finish the refactor and submit that.

Specific notes

Contributors other than yourself, if any: @wwieder

CTSM Issues Fixed (include github issue #): None

Are answers expected to change (and if so in what way)? Adds attributes to write_climo files' dimension variables.

Any User Interface Changes (namelist or namelist defaults changes)?

• Adds --output-dir option. Default ./anomaly_forcing reproduces previous behavior.
• Makes synonyms for options with hyphens replacing underscores.

Testing performed, if any: Compared files for (a) write_climo, (b) tas, and (c) WIND generated with 389bca9 (only superficial changes) and 2b45ef4 (latest commit on branch).