Boundary conditions for MHD

Mentioned by @andrewwinters5000 in the first post of #636:
> Only periodic BCs are currently supported by `TreeMesh` for MHD, so adding BCs would require a modification of `calc_boundary_flux!` for all solvers to include a check for `have_nonconservative_terms(equations)`

Start by locating calc_boundary_flux! and the handling of have_nonconservative_terms(equations) across the solvers. Clarify which non-periodic boundary conditions and solver coverage are required, then verify the implementation with tests demonstrating the requested MHD boundary behavior.

Written by the indexing model from the issue text.