Allow for the processing of seasonal datasets

[monocongo]

Is your feature request related to a problem? Please describe. Users have requested an ability to process seasonal datasets.

Describe the solution you'd like A new --periodicty argument for "seasonal" (DJF, MAM, JJA, SON)

Describe alternatives you've considered It may be possible to convert data on-the-fly from monthly or daily to seasonal using pynco, xarray, or pandas for resampling.