Add algorithm `forEachBoundaryDOF()` and class SubEntityDOFs

The algorithm allows to iterate over all DOFs of a given global basis associated to sub-entities located at the boundary of the domain. This also demonstrated the usage of this algorithm for the boundary treatment in the poisson-pq2 example.

It is based on the new class SubEntityDOFs. After binding an object of this class to a local view and a sub-entity it can be used a range of local indices of DOFs associated to sub-sub-entities of this sub-entity.

While the algorithm simplifies elementar boundary treatment (e.g. mark all boundary DOFs) a lot, the SubEntityDOFs range can be used to implement more complicated stuff, e.g. if different leafs in the ansatz tree should be treated differently.