Add capability to set the layers.
Revert change for setting layers.
Add top_bottom_bcs flag to IterationSpace.
First version of LHS top and bottom boundaries in extrusion.
Only apply the assembly with bcs on the LHS.
No flattened version of the bcs assembly is needed.
Change assemble_bcs flag structure.
Eliminate the top_bottom_bcs flag from the IterationSpace.
Fix the assemble_bcs flag.
Remove the top_bottom_bcs flag from Set.
Change the cache key.
Use empty list.
Change the local tb variable to top_bottom.