Use comm.scatter for mesh distribution

[inducer]

• cf. Add new helper for distributing data across MPI ranks meshmode#342.
• goes with (but doesn't require) Deprecate MPIMeshDistributor meshmode#376.

[majosm]

Hmm, I think the mirgecom tests are failing because the wall coupling was merged (and therefore mirgecom now depends on my #285 branch). I'm going to check if there's an easy way to make those changes conditional so the CI still works here in the meantime.

[inducer]

No need IMO, as long as #285 goes in soon. I can just ignore the mirgecom failures for the time being. But I've put the downstream issue on the agenda for Friday, too, to at least have a discussion.

[majosm]

LGTM.