Add top and bottom facet integration for zero forms.
Remove iteration set flags and add iteration descriptors

Add the iteration descriptor to the loop cache key.
Add support for iteration over the interior facets by limiting the number of layers.
Stage in map for second vertical cell using current cell map and offset.
Attempt to add support for interior vertical facets
Fix top facet maps in wrapper to include size of the offset
Sparsity handles ON_BOTTOM, ON_TOP, ON_INTERIOR_FACETS

Only for the non-MPI case.
Interleaved stage-in in the wrapper for interior horizontal facets.
Support for interior facets.
Move row dimension computation inside the loop.
Fix the SparsityMap type to work with MixedMaps
Fix the RHS extruded map offset for top facet integrals.
FFC interface: iterate over integrals in preprocessed form
Fix number of parameters to not include layers in non-extruded case
Fix the it_space for Map objects.
Remove the underscore from the offsets over the interior facets.