[BDFM] implement BDFM cube finite elements
The coefficients and interpolation are implemented for arbitrary dimension (>1) and order (>0). The actual basis is only implemented for dim =2 and order=1,2,3.
The basis was calculated with base.py
Note:
this MR depends on dune-common!471 (merged) for binomial(...)
Note to the interpolation:
The unrank(...)
algorithm and its inverse rank(...)
(not implemeted here) might be better moved into dune-common!471 (merged).
The split into trace and interior interpolation is the interface that I actually need for my code.