MPI FiniteDifferences

It would be nice to have a new `FinitedDifferences`-like class based on [`mpi4jax`](https://mpi4jax.readthedocs.io/en/latest/), to enable HPC applications.

A good milestone could be to reproduce the [shallow water](https://mpi4jax.readthedocs.io/en/latest/shallow-water.html) example of the library (original code [here](https://github.com/mpi4jax/mpi4jax/blob/master/examples/shallow_water.py))