Simplify PETScMatrix::zero

Internally MatZeroRowsIS used MatZeroRows anyway
Fix DirichletBC::zero in parallel

requires additional function zero_local in GenericMatrix interface
Fix DirichletBC::apply in parallel when use_ident=false
Add scripts for running tests with different granularity under gdb and mpi repeatedly for debugging for deadlocks.
Use fixture for parameterization in new test.
Add back verbose output to pytest targets in cmakelists, we may need this info on the buildbot. Probably possible to make pytest dump this to log files which would be better.
Edits to the mpi test running scripts.
Add new test in new file prior to merging with pytest tests in master branch.
Remove test added as another file.
Merge branch 'master' into mliertzer/fix-dirichletbc-zero
Move new test into test_dirichlet_bc.py.
Fix warnings.
Minor edits in mpi running scripts.
Disable test_ident_zeros in parallel to see if this makes the tests stabilize on the buildbot.
Bugfix in test skip.
Merge branch 'martinal/topic-mpi-test-debugging'