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 !52 (merged))
- IIRC it entered dune-common without discussion.