Move inclusion of petsc4py.i to the common module forcing it to be imported in all modules.
This forces the petsc4py typemaps for MPI_Comm to be used throughout the whole DOLFIN library
Add simple test of MPI_Comm when compiled with petsc4py
Do not reconstruct the cpp.FunctionSpace when turning a cpp.FunctionSpace from C++ to Python
Merge branch 'johanhake/fix-issue-197'