Add LFEPreBasisMixin class

Summary

Add mixin class LFEPreBasisMixin to create leaf pre-bases based on a local finite-element and a DOF layout. This allows to implement several simple global finite element spaces based on their analogon in dune-localfunctions. It requires that the global basis does not require any reordering of DOFs on entities. This cannot currently be handled by the MCMGMapper class, but might be possible in the future.

TODO

• Merge !433 (merged) to make the test pass.