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
Merge branch 'johanhake/add_vertex_assembler'
Move test to new position
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.
Merge branch 'mliertzer/fix-dirichletbc-zero' into next