Add rf_local_is_in function

[vpipkt]

Add function to return an array indicating if cell values are in a provided array or python list.

Update docs to show this in masking

Also resolving these:

Close #351 Close #310

[vpipkt]

Docs as rendered: https://1690-174014757-gh.circle-artifacts.com/0/rf-site/supervised-learning.html#visualize-prediction