Merge branch 'feature/matrix-inversion' into 'master'

ref:dominic/dune-perftool Matrix inversion at code generation time does only
work to a very limited extent (up to n=4). We can instead assemble the tensor
in C++ and invert it there (e.g. using Dune::FieldMatrix)

This fixes [#123].

Still TODO:

-   \[ \] Vectorized Inversion

See merge request [dominic/dune-perftool!263]

  [#123]: gitlab.dune-project.org/NoneNone/issues/123
  [dominic/dune-perftool!263]: gitlab.dune-project.org/dominic/dune-perftool/merge_requests/263


Closes #123