VTK: Did not clear label before writing
MeshIO Tests: Changed what seem to be clearly wrong test data
Merge branch 'knepley/upgrade-petsc-interface' of github.com:geodynamics/pylith into knepley/upgrade-petsc-interface

# By Brad Aagaard
# Via Brad Aagaard
* 'knepley/upgrade-petsc-interface' of github.com:geodynamics/pylith:
  Added MeshOps::checkTopology().
  Trivial code cleanup.