Add parallel test of bc application
Use new inc_local_diagonal_entries to set bc nodes in Matrix

In parallel, we can't use zero_rows since the row indices are sent
through a map.  Instead, given that we only actually need to set a value
on the diagonal, use new PyOP2 functionality to do that.
Merge pull request #158 from firedrakeproject/fix/parallel

Fix setting diagonal entries of matrices for boundary condition
application when running in parallel.