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.

mentioned in merge request !52 (merged)

Objections?

Last call for objections!

Mentioned in commit 49440f1c

Status changed to merged

Mentioned in merge request !106 (closed)