Add (optional) arbitrary region support with regionmask

[blychs]

Dear all,
If we allow people, as an optional dependency, to include the package "regionmask", we could easily allow for using arbitrary shapefiles, counties, etc.
https://regionmask.readthedocs.io/en/stable/
If you are interested, I should be able to add support for it.
Cheers
Pablo

[blychs]

Hi all,
I know that this is strongly wished for by NCAR. @rschwant , would it be OK for me to move forward?
Otherwise, I can to keep it in a different fork.
Cheers
Pablo

[rschwant]

Yes, sorry this got lost in my emails. Please proceed forward with this.

We do have filtering options already, so just make sure to consider these filtering options while you are coding this in.
https://melodies-monet.readthedocs.io/en/stable/appendix/yaml.html (search for filter_dict) which can be applied in the observations or in the data_proc section for plotting and statistical calculations.