Pass down the cell and facet domains into the facet wise assembly loops so that sub domain information may be used.
Merge branch 'master' into assembly
Rewrite of SystemAssembler facetwise assembly using the cell and exterior facet domains.
Merge in master and resolve conflicts (accept theirs).
Merge branch 'tmpassembly' into assembly
Reorder fetching the default versus domain integral in case there is no default integral but only domain specific ones.
Add a very simply test.

Assemble a 2d p1dg mass matrix over two subdomains with different coefficients in each.  Assemble the rhs with the same
coefficients.  Solve.  Check the answer's unity.

Repeat but add in a dummy internal facet integral to test facet wise assembly as well as cell wise.
Merge branch 'master' into assembly
Merge branch 'master' into assembly
Merge branch 'master' into assembly
Remove some const on return types - picked up by Intel compiler.
Some more const MPI_Comm fixes.
Merge branch 'cian/fix-sym-assembler-subdoman-info'