Handle idomain and inactive cells

[p-ortega]

Potentially some array house keeping would have to be done to accommodate models with inactive cells. Let's see

[jtwhite79]

@p-ortega for the adjoint stuff, we found that if there are inactive nodes, there is a nodeuser and nodereduce array that can be used to map from a full list of nodes to the active nodes and back...is that what you are seeing also?

[p-ortega]

Hey @jtwhite79

Good call!! I'll try that and see how it goes.

Actually I'm seen this issue in a model for a client (MODFLOW-ORG/modflow6#2021)

I open this issue here to not forget to bulletproof mf6rtm.