Motivation

Currently, MFEM's DG kernels and boundary conditions are not supported.

Design

Add extra containers to the Moose::MFEM::EquationSystem to hold the DG kernels and the DG boundary conditions. Adding both comes through MFEMProblem, much like regular kernels and boundary conditions. Adding DG Kernels from the input file reuses the existing Moose DG kernels machinery (this PR includes an overload for addDGKernel). Adding DG Boundary conditions gets its own action, add_mfem_dg_bc.

Impact

Most changes are internal Moose::MFEM::EquationSystem, with the addition of one new action.