This should allow for .ccs with same name
in different dirs.

* we add the include dirs and link dirs per grid type
  depending only on the fact, that the grid was found
* we also add the libs of all found grids to GRIDLIBS
* depending on the users choice of GRIDTYPE we only
  set stuff like -DENABLE_FOO for the
  Dune::GridSelector, nothing else
All in all, this should finally allow to use different
grids in one place (e.g. lib).

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We use it all over the place. Adding a guard everywhere
would be insane.

mpihelper.hh is moved in dune-common >= 2.3

By now we make use of it in several places.

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Conflicts:
	dune/stuff/grid/search.hh