allow the opm-parser test to fail

since the module currently calls the tests for cJSON and boost with
the REQUIRED flag set, cmake aborts completely even if opm-parser is
not REQUIRED. For modules which depend on opm-core that's currently a
non-issue because core has a requirement on parser, but the parser may
be useful for external projects as well, so we play nice.

Also, with this patch the config.h variable HAVE_OPM_PARSER is set...
[buildsystem] Make dune-cornerpoint a suggestion.
do not add the "-Wl,--as-needed" linker flag in FindSuiteSparse.cmake

because if umfpack is fully linked but some other library is
underlinked, it causes the build to fail. This flag is better dealt
with in the "UseOnlyNeeded.cmake" module...
make the "-Wl,--as-needed" flag optional

as there is not much harm done if it is not used but it may cause the
build to fail if it is present (read: it causes _my_ build to fail),
it defaults to OFF.
Fix option name to match name in use in conditional.

Also add a useful doc string.
Merge pull request #580 from atgeirr/fix-external-warnings-option

Fix option name to match name in use in conditional.
Merge pull request #577 from andlaus/make_as_needed_optional

Make as needed optional
Merge pull request #576 from bska/sync-build

[buildsystem] Make dune-cornerpoint a suggestion.
Added function grid_equal() to compare two UnstructuredGrid instances.
Renamed struct EclipseGrid -> struct EclipseWriterGrid to avoid conflict
with class in opm-parser.
ERT-601: Added GridManager() constructor which takes a Parser::EclipseGrid instance
Merge pull request #574 from andlaus/allow_opm-parser_to_fail

allow the opm-parser test to fail
Changed the data files used for testing:

1) Added RUNSPEC section
2) Replaces DEPTHZ keyword with TOPS keyword.
Add missing break statements in switch, add default case.
Merge pull request #582 from atgeirr/fix-switch-fallthrough

Add missing break statements in switch, add default case.
Merge pull request #581 from joakim-hove/GridManager-EclipseGrid-constructor

Grid manager eclipse grid constructor
convert the EclipseGridInspector to opm-parser

that one is completely unused in opm-core (*booo*!), but used quite a
bit in dune-cornerpoint and dune-porsol, so let's keep it for now...
CornerPointChopper: add methods opm-parser'y methods
add opm-parser variants for the incompressible property classes
remove the import_rewrite example

that one was quite specific to the old parser and would thus require
more work to convert to the opm-parser than what it is worth the
effort. If you _really_ want to keep this tool, ping me and I'll port
it...