[cmake] include eigen as system header
[dune.module] suggest dune-geometry
By now we make use of it in several places.
fix mpihelper include
mpihelper.hh is moved in dune-common >= 2.3
[dune.module] require dune-geometry
We use it all over the place. Adding a guard everywhere
would be insane.
added missing HAVE_DUNE_GRID guard