mmc is broken

[LourensVeen]

Describe the bug
mmc's AMUSE integration is quite weird, and it doesn't have tests so we're not sure it works. There's code generation (and a promise to remove it), an AMUSE wrapper that itself is a separate worker, and the whole thing is in all-caps Fortran 77.

Because of this I skipped it when implementing the new build system (#1024). It's probably fixable by essentially re-coupling the code, but it seemed like a lot of work so I've parked it for now until there's a clear use case.