[buildsystem] Make dune-cornerpoint a suggestion.
Added implementation of free function interface grid interface for CpGrid.

This implements a superset of the interface as proposed in pull request
 opm-core#496 for use with CpGrid.
It also adds some additional functionality needed in opm-autodiff.
Removes the dependency of FullyImpliciteBlackoilSolver onto UnstructuredGrid.

With these changes it will be possible to use CpGrid with FIBOS except for the
output routines.
Add an example program of FIBOS that uses CpGrid.
Added function for querying the face area to the helpers.
[bugfix] Fixes elementAt of CentroidIterator.
Removed last occurence of the grid adapter in the EclipseOutputWriter.
Implemented VTK output for CpGrid using DUNE's VTKWriter and activated Matlab for CpGrid.
Merge remote-tracking branch 'upstream/master' into master-refactor-for-cpgrid-support

Manually resolved conficts in the following files
	examples/sim_fibo_ad.cpp
	opm/autodiff/FullyImplicitBlackoilSolver_impl.hpp
	opm/autodiff/SimulatorFullyImplicitBlackoil_impl.hpp

In additions examples/sim_fibo_ad_cp.cpp was adapted to compile again.
Activate paralle istl solvers if there is more than one process computing.
Merge remote-tracking branch 'upstream/master' into master-refactor-for-cpgrid-support

Conflicts:
	examples/sim_fibo_ad.cpp
	opm/autodiff/FullyImplicitBlackoilSolver_impl.hpp
Merge remote-tracking branch 'upstream/master' into master-refactor-for-cpgrid-support

Resolved conflicts:
	opm/autodiff/FullyImplicitBlackoilSolver_impl.hpp
Merge branch 'master' into blattms-master-refactor-for-cpgrid-support

Conflicts:
	examples/sim_fibo_ad.cpp
	opm/autodiff/FullyImplicitBlackoilSolver_impl.hpp

This brings the "CpGrid support" branch up to date with respect to
recent changes in opm-autodiff master.
Updates interface to fix issues not resolved in the latest merge.

Namely the template parameter was missing for the added methods.
In addition there were still various instances where the grid was
used directly rather than via the functions in GridHelpers.hh.
Adapted sim_fibo_ad_cp with latest changes for sim_fibo_ad
Revert "Activate paralle istl solvers if there is more than one process computing."

This reverts commit 9598a990df2691196ecd68add8dc0667a86983dd.
The commit slipped in by accident and should be part of a separate branch.
Merge branch 'tester' into master-refactor-for-cpgrid-support

Resolved conflicts:
	examples/sim_fibo_ad.cpp
	opm/autodiff/FullyImplicitBlackoilSolver_impl.hpp
Updated examples/sim_fibo_ad_cp.cpp according to latest changes in examples/sim_fibo_ad.cpp
GridHelpers: Fix build when !HAVE_DUNE_CORNERPOINT

This commit fixes an incorrectly nested "#ifdef..#endif" block that
leads to build failures when opm-autodiff is being used without
dune-cornerpoint.
Bring a definition of OPM_THROW into scope.

The template version of outputStateMatlab() uses OPM_THROW so we need
a definition of the macro.