Allow custom communicators

[matz-e]

This is technically not as trivial as expected, since it requires something along the lines of

#include <mpi4py/mpi4py.h>
 
namespace pybind11 { namespace detail {
  template <> struct type_caster<mpi4py_comm> {
    public:
      PYBIND11_TYPE_CASTER(mpi4py_comm, _("mpi4py_comm"));

      // Python -> C++
      bool load(handle src, bool) {
        PyObject *py_src = src.ptr();

        // Check that we have been passed an mpi4py communicator
        if (PyObject_TypeCheck(py_src, &PyMPIComm_Type)) {
          // Convert to regular MPI communicator
          value.value = *PyMPIComm_Get(py_src);
        } else {
          return false;
        }

        return !PyErr_Occurred();
      }

      // C++ -> Python
      static handle cast(mpi4py_comm src,
                         return_value_policy /* policy */,
                         handle /* parent */)
      {
        // Create an mpi4py handle
        return PyMPIComm_New(src.value);
      }
  };
}} // namespace pybind11::detail

See: https://stackoverflow.com/a/62449190

The issue is that it requires that we build our C++ against the C++ backend of mpi4py. Which opens us up to incompatibilities, e.g., because pip install mpi4py installs one version but CMake picks up an entirely different version.