Update to Python unit tests for C++ interface changes.
Merge branch 'knepley/upgrade-petsc-interface' of github.com:geodynamics/pylith into knepley/upgrade-petsc-interface

* 'knepley/upgrade-petsc-interface' of github.com:geodynamics/pylith:
  Update to Python unit tests for C++ interface changes.
Updated Python unit tests for interpolated meshes.

Filter out faces and edges when writing groups.