Merge branch 'feature/permuting-horizontal_adds' into 'master'

ref:extensions/dune-codegen The number of alternative implementations will
grow in the near future, so its much better to have them in dune-codegen than
patching the VCL\...

See merge request [extensions/dune-codegen!309]

  [extensions/dune-codegen!309]: gitlab.dune-project.org/extensions/dune-codegen/merge_requests/309