Remove dublicate EXTRA_DIST line.
Do not pass reference to local objects.

Clang warns about it.
Merge branch 'releases/2.3'

Pull in Christoph's fixes from release.

* releases/2.3:
  Do not pass reference to local objects.
  Remove dublicate EXTRA_DIST line.
Get rid of a bunch of compiler and deprecation warnings.
Introduce subEntities() and deprecate count() in Entitiy.

Needed to pass gridCheck.
Fix use of ALUGrid, add suggestion to dune-alugrid.