Add initial test for facets.
Add measures to core types.
Add support for top and bottom facet zero form integrals.
Add bottom set to _Facet objects.
Remove iterset flags and use iteration descriptors.
Add support for extruded exterior facet integrals.
Add another extruded facet test. Fix issue with exterior vertical facets.
Add test for vertical facet subdomains. Clean up code.
Add horizontal facet integral with temporary fix for avoiding double jacobian kernels.