Require non-trivial distribution only for recent Dune

The CpGrid::scatterGrid() method produces non-trivial results only
if we have MPI support *and* if the target Dune platform is
sufficiently recent (i.e., dune-grid >= 2.3).  Therefore, we cannot
"REQUIRE" non-trivial results unless both conditions are satisfied.

In the process, also extend the diagnostic in CpGrid::scatterGrid()
to include information about the dune-grid requirement.

This fixes "make test" in configurations with MPI and Dune 2.2.1.
Merge pull request #77 from bska/loadbalance-dune221

Require non-trivial distribution only for recent Dune
Add missing forward declaration of CpGridData.
Merge pull request #78 from atgeirr/add-missing-forward-decl

Add missing forward declaration of CpGridData.
Suppress unused argument warnings.

Done with the (void) trick in a non-MPI block, since functions do use
their arguments in the MPI case.
Comment out unused argument.
Renamed i->j to avoid shadowing.
Merge pull request #79 from atgeirr/suppress-warnings

Suppress warnings