add regionmask

[alaniwi]

Package name regionmask

https://anaconda.org/conda-forge/regionmask

user requested (part of HS ticket 57433)

Details from requesting user:

• used to create masks of gridded data
• details at https://regionmask.readthedocs.io/en/stable/index.html

[alaniwi]

conda package regionmask

[alaniwi]

dup #98