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.
Fix solving elif error.
Attempt to add interior vertical facets.
Interior facet integrals for zero forms.
Split tests on extruded facets.
Solve a Poisson equation on exterior top and bottom domains.
Add Poisson test with top and bottom integrals.
layers -> layers - 1
Revert integral mismatch fix. Add bottom_set to _Facets.
Facets work for new version of PyOP2 and Firedrake.
adding some functionality tests (but not correctness tests)
Add changes to support the sparsity building for horizontal facets.