Multigrid PDE solver

Walberla should eventually have a multigrid PDE solver. This should also enable solving PDEs on refined grids. As requested in software/walberla#15, it should optionally be possible to specify the full sparse matrix for the left-hand side instead of just giving stencil weights.

Strategy:

1. Basic multigrid with direct coarsening and no boundary conditions

• Add stencil field support to existing PDE solvers (software/walberla#15), as this is required for the following steps.
• Implement Galerkin Coarsening
• Locally modify the stencil to take into account boundary conditions
• Grid refinement
  • Make sure RBGS work on refined grids. We can assume that all blocks have the same coarsest resolution.
  • Adapt V-cycle so that for its finest levels, it can use the block's resolution if no grid with the needed resolution is available.
  • For dynamic refinement, need to recreate the coarser levels of the V-cycle (solution, rhs, residual, stencil field) whenever the refinement changes.