Fix issue with jit compilations on different mpi groups
More fixes for mpi_comm world and jitting.
 -- Still need some tests...
Improve support for mpi groups and jit compilation.
  -- We need to initialize PETSc manualy for this to work
Remove old test in test_jit.py
Merge branch 'johanhake/mpi-group-support-in-jit_compilation'