[WIP]: Add algorithms for working with function space bases

This is a collection of algorithms working on function space bases. It does e.g. allow to

• loop over all (local view, local index set) pairs for a given basis
• loop over all DOFs associated to an intersection
• loop over all boundary DOFs

This is not realized by fixed utility methods that do, e.g., mark the DOFs. Instead these are stl-like algorithms invoke given callback functions. This allows for greater flexibility and composability.

The namespace TreeAlgorithms contains some algorithms for looping over a type tree. These are proposed for dune-typetree. However I copied them here to simplify the implementation. They can be removed once they have arrived in dune-typetree.

Furthermore the algorithms for looping over sub-sub-entities of a sub-entity in a reference element may be moved to dune-geometry.