support periodic boundary conditions for self-gravity

[BenWibking]

Describe the proposal
We currently only support open boundary conditions for the Poisson equation using the amrex::OpenBC Poisson solver. We can also use amrex::MLMG to do the Poisson solve for periodic BCs.

Describe alternatives you've considered
N/A

Additional context
Needed for cosmological PopIII stars.

[BenWibking]

@markkrumholz IMO this could be another good intermediate project to tackle before adding AMR subcycling support to the gravity solver, since using the amrex::MLMG class will be necessary for the Poisson solves on all refined AMR levels.

[markkrumholz]

@lizmcole, @BenWibking: that does sounds like a good idea, and a necessary step before going to adaptive time stepping anyway.