make MPI optional through julia extension

[mloubout]

Move all the mpi code into an MPI extension that only gets loaded when using MPI is called. This will allow to use serial devito without having to worry about an MPI preferences or library.

MPI extension is tested as well.

All tests passs locally, might need few tweaks for CI

Testing decoupler as well

[samtkaplan]

Thanks @mloubout , this is looking great. one question: can we remove MPIPreferences from the deps? In my quick scan through the changes I don't think it is needed, but I may have missed something. Thanks.

[mloubout]

can we remove MPIPreferences from the deps?

I'm not sure. It's needed because of the extension and extensions don't accept dependencies. So to be able to set the preference for the extension the only seems to be to keep it. Not sure what a woraround would be.

Concerning the Pro/decoupler tests, i think it will only be tested once merged because the secrets are (rightfully) locked to you guys only, so you might wanna check the pro tests still work on your side.

[samtkaplan]

@mloubout I made an alternative PR over here: #207. I'm not sure how to add you as a reviewer to that one, but if you have time I would appreciate a look.