[!295] Feature/loopy invert matrix

Merge branch 'feature/loopy-invert-matrix' into 'master'

ref:extensions/dune-codegen Moves the matrix inversion stuff from
blockstructured/geometry.py to pdelab/tensor.py. This touches on the subject
of [#139].

You can get the name of the inverse of a matrix A with shape shape from

    name = name_matrix_inverse(A, shape, visitor)

matrix_inverse() returns the inverse wrapped in a pymbolic variable, although
I don't know how useful this is.

TODO:

-   [x] handle inames correctly
-   [ ] what about tags? -\> not really necessary
-   [ ] what about dependencies -\> not really necessary

See merge request [extensions/dune-codegen!295]

  [#139]: gitlab.dune-project.org/NoneNone/issues/139
  [extensions/dune-codegen!295]: gitlab.dune-project.org/extensions/dune-codegen/merge_requests/295