Possible extensions

Ideas for extending this Poisson tutorial:

• unstructured grid, maybe read from a file. This relates to the grid I/O tutorial and uses other grid managers than YaspGrid.
• using higher-order finite elements, e.g. Q2 or P2, with additional DoFs on edges (or even in the element interior). This requires a mapper, as introduced in the corresponding grid data tutorial.
• non-homogeneous boundary conditions, e.g. a subset of the boundary with Dirichlet conditions and the rest Neumann, possibly using boundary intersection normal vectors.
• build a local element matrix and local element vector before scattering into the global matrix.
• thread-parallel assembly.