Add test for BrokenElement and TraceSpace functionality
update TraceSpace -> TraceElement
Pin down exterior facet trace dofs by applying zero BC
Merge pull request #320 from firedrakeproject/hybridisation_test

* hybridisation_test:
  Pin down exterior facet trace dofs by applying zero BC
  update TraceSpace -> TraceElement
  Add test for BrokenElement and TraceSpace functionality