add interior facet stress tests (steady advection)
Eliminate extra global args not needed for new FFC kernel types.
Use ON_TOP, ON_BOTTOM instead of magic numbers.
Eliminate bottom_set from _Facets constructor. Drop use of kwargs.
Fix flattening of the coefficients.
Add some P1DG steady advection tests

Conflicts:
	tests/regression/test_steady_advection_3D.py