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. [x] Basic multigrid with direct coarsening and no boundary conditions * [x] Add stencil field support to existing PDE solvers (software/walberla#15), as this is required for the following steps. * [x] Implement Galerkin Coarsening * [x] 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.