Merge branch 'feature/bdfm-spaces' into 'master'

ref:core/dune-localfunctions 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] for binomial(...)

##### Note to the interpolation:

The unrank(...) algorithm and its inverse rank(...) (not implemeted here)
might be better moved into [dune-common!471].

The split into trace and interior interpolation is the interface that I
actually need for my code.

See merge request [core/dune-localfunctions!105]

  [base.py]: /uploads/79e53ef938abe294040b9fb4404d05f3/base.py
  [dune-common!471]: gitlab.dune-project.org/Nonedune-common/merge_requests/471
  [core/dune-localfunctions!105]: gitlab.dune-project.org/core/dune-localfunctions/merge_requests/105