Rename remove_nan_layers to 'remove_inactive_layers' or 'remove_zero_thickness_layers'

[bdestombe]

          we should probably rename this to 'remove_inactive_layers' or 'remove_zero_thickness_layers'.

With the move to internally always assume modflow-style layer models (with one top and N bottoms), renaming this kwarg would make it more clear what we're doing.

We do need to maintain some backward compatibility though, so we should add the new name and throw a DeprecationWarning for the old one. Up to you whether you open a new issue for this or add it as part of this PR :).

Originally posted by @dbrakenhoff in #295 (comment)