This has just come through from the original code.  However, I note that it may not do what we want.  Doru, you set this flag on the Set to indicate that you want to apply bcs to the top and/or bottom.  How does this play if you have multiple par_loops (lazy) where the first has some bcs and the second doesn't.  Should this instead (like for side facets?) be a property of the iteration space and come in via a keyword argument to the par_loop?

If yes, I think we can leave this as is for now and fix it up in the putative extruded facets merge.