Don't include "config.h" in public header

That's always an error.  Presumably, this was a left-over from
conversion to templates.
Prefer C++ headers

Also, remove a couple of headers that were no longer needed.
Implement Dune::CpGrid operations when needed only

The functions

    multiplyFaceNormalWithArea
    maybeFreeFaceNormal

operating on Dune::CpGrids must not be implemented unless they are
actually used.  Otherwise we get link failures if (e.g.,)
opm-autodiff is being built without CpGrid support.

Hide the definitions behind "#ifdef HAVE_DUNE_CORNERPOINT".
Declare initBlackoilStateFromDeck(DeckConstPtr)

This was missing in commit 4c2120c and produced some build failures
that were hard to analyse.  I'm not convinced that the underlying
problem is solved, but this does at least restore the build.
Merge pull request #2 from bska/master-refactor-for-cpgrid-support

Fix build in !HAVE_DUNE_CORNERPOINT case