Local consistency and conservation tests for remapping operators

Local conservation: [Ullrich & Taylor](https://journals.ametsoc.org/view/journals/mwre/143/6/mwr-d-14-00343.1.xml) pt 1 eq 22
Will directly test the `overlap` function that generates the local overlap weights.
Checks that the weights from a subset of the source grid dof are conserved in the target mesh.

Local consistency: Ullrich & Taylor pt 1 eq 23
Checks that for a subset of the target grid dof, the row sums of the remapping operator are 1 if the corresponding dof is in the set, and 0 otherwise.

Testing local submaps will enable checks that remapping operators built for non-overlapping grids are correctly constructed.