[ ] Go through system part -> GFS
[ ] Export boundary grid function

IT is both simplicial and quadrilateral...

So far, we were a bit sloppy about which basis we choose from the UFL
counterpart. This commit implements the necessary 1-to-1 mapping.

In many places we treat this as codegen-time information anyway.
For loopy, this is the necessary way to go.

This reverts commit 6a1e7cc9

This reverts commit f83a804e

I've already mentioned this problem in [!314] but I think more
visibility for this problem is useful