Cookie cutting mask generation

[benjaminleighton]

Integrate https://github.com/CSIRO-enviro-informatics/c3dis-2019-pangeo/blob/master/All_Catchment_Polygon_Mask_Averages.ipynb and (preferably) salem base approaches to cookie cutting / producing masks.

I suggest the first iteration on this might be to add new functions to functions.py that accept geojson or files, a target xarray template (or maybe another kind of description of spatial dimensions) and generates a mask xarray. Additional functions might save the mask to disk and load and apply the mask to real datasets.