Merge branch 'feature/finite-element-implementation-selection' into 'master'

ref:extensions/dune-codegen So far, we were a bit sloppy about which basis we
choose from the UFL counterpart. This commit implements the necessary 1-to-1
mapping.

This fixes [#146]

See merge request [extensions/dune-codegen!345]

  [#146]: gitlab.dune-project.org/NoneNone/issues/146
  [extensions/dune-codegen!345]: gitlab.dune-project.org/extensions/dune-codegen/merge_requests/345


Closes #146