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.
Restore missing constructor argument

The 'grav' argument (gravity vector) was apparently lost in a
mis-merge.  This commit restores the missing argument.
Annotate end of namespace and "#if"

This commit adds a few annotations to closing braces on namespaces
and one "#ifdef" conditional.  This is an aid to (hopefully) avoid
the problem fixed in commit 688d65e.
Use full path in examples_SOURCES

That's the convention used elsewhere in the build system and we
should honour that convention.  While here, remove the assignment to
PROGRAM_SOURCE_FILES because that variable is no longer referenced
once we start to run the hooks.
Support older versions of Dune

Retrieving the "mpihelper.hh" from <dune/common> rather than
<dune/common/parallel> means we can still support (e.g.,) Dune 2.2.1
if needed.
Use new parser for all deck information

This commit switches to using the convenience method

    GridManager::createGrdecl()

to construct a "struct grdecl" that can be passed directly into
CpGrid::processEclipseFormat().  The practical benefit is that we
get to leverage the opm-parser that's in active development rather
than the calcifying "EclipseGridParser" that will only become less
relevant in the future.  We also don't parse the data twice.

Note: This is mostly compile tested.  In the few actual tests I ran,
I got into trouble with internal assertions in processEclipseFormat.
That problem is not yet analysed.
Fix memory leak

Function GridManager::createGrdecl() generally allocates memory
(using std::malloc()) for the MAPAXES keyword data output of the
'grdecl' structure.  Release those resources before leaving the
scope to prevent memory leak introduced in commit fdca540.
Merge pull request #2 from bska/master-refactor-for-cpgrid-support

Fix build in !HAVE_DUNE_CORNERPOINT case