Merge branch 'feature/deprecate-identitymatrix' into 'master'

Deprecate the identitymatrix.hh file

This patch makes the preprocessor emit a #warning whenever the file
identitymatrix.hh is included.

Reasons:
- It is not used anywhere in Dune core or staging or any other module I know.
- I have never heard a compelling reason to have it.
- It produces maintenance costs
  (see core/dune-common!52)
- IIRC it entered dune-common without discussion.

See merge request !136