Rework support for non-overlapping computations

Non-overlapping computations in PDELab right now depend on passing a GridView for the InteriorBorder partition to the function space. This functionality will be deprecated in Dune 2.4, so we need to come up with an alternative. I think the cleanest way of handling this is to wrap the GridView passed into the function space. That will also allow us to supply a custom IndexSet that is restricted to the InteriorBorder partition, which gets rid of a lot of special-casing to work around problems due to "holes" in the index set when using anything but the All partition.

I'll cook up an initial prototype and post a merge request later this week.

There is now a mostly-ready implementation in !69 (merged).

Status changed to closed by commit 5f0d7cd7