Better diy-mpi interface

1. In header-only mode, non-explicit conversion functions between
DIY_MPI_..., MPI_... and diy::mpi... types are available. So passing MPI types
directly where DIY_MPI... types are expected should work.

2. In library mode, use `mpi_cast` to convert from diy to mpi types and
`make_DIY_MPI..` to convert from mpi to diy handle types.

3. In code that is expected to work in both header-only and library
modes, method 2 above should be used. This includes all code that is
part of the diy library.