add option to quell dune(-istl) warnings
add support for the UG grid library

recently UG has become free software (yay!), so we can now support it
with a good conscience. The CMake module is based on the "UseUG.cmake"
module of dune-grid but it is quite a bit simpler and actually
works. (I cannot see how UseUG.cmake can work without ever including
the ug-config*.cmake files...)

the UG source code is available here:

https://github.com/ugfem/ug
Comment or add (void) usage of unused arguments.
Avoid signed/unsigned mismatch.
Comment out unused function argument.
Avoid signed/unsigned warning.
Add (all-zero) initialisation of new EquilRecord fields.
Merge remote-tracking branch 'akva2/quell_dune_warnings' into suppress-external-warnings
Fix access to keywords, while still silencing warning.

Previous fix was wrong because it called getKeyword() outside the
hasKeyword() check. Current version (like original code) uses the
short-circuiting behaviour of && to ensure this.
Added more warnings to suppression list, sorted list.
Make function in header file inline.
Merge branch 'suppress-warnings' into suppress-external-warnings
Change option name and generated header names.
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...
Merge pull request #571 from atgeirr/suppress-warnings

Suppress warnings
Merge pull request #575 from atgeirr/suppress-external-warnings

Suppress external warnings
Merge pull request #555 from andlaus/add_ug_support

add support for the UG grid library
[buildsystem] Make dune-cornerpoint a suggestion.
add the missing TABDIMS keyword to the live oil and wet gas unit test data files

these are required for the new parser. (without them, these files
would be invalid AFAICT, but I'm not an Eclipse file format expert...)
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...