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.

dune/common/identitymatrix.hh

 #ifndef DUNE_COMMON_IDENTITYMATRIX_HH
 #define DUNE_COMMON_IDENTITYMATRIX_HH
 
+#warning Deprecated since dune-common 2.5: If you really do need an identity matrix, use DiagonalMatrix or ScalarIdentityMatrix (from dune-istl) instead!
+
 #include <dune/common/boundschecking.hh>
 #include <dune/common/fmatrix.hh>
 #include <dune/common/ftraits.hh>