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

A pattern that we have been manually implementing in quite some places!

This is not used anymore since we have mixins.

In the /sumfact_poisson_dg_3d_unstructured_deg2_symdiff_nonquadvec_gradvec
test case this reduces the time spent in evaluation of it from >100s to 6s.

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 05a2c7d3

This reverts commit 6a1e7cc9

This reverts commit f83a804e