Python wrapper code for custom casters (mpi4py, petsc4py) does not work with JIT-ed code

[Tormod Landet]

We only got this working for code that is compiled along with the dolfin pybind11 wrappers. When code is JIT-ed afterwards the custom caster and wrapper are not included and the automatic conversion will fail

See the xfail in the unit test for a case that should be made to work