Glue in support for the grid property modifier keywords

this basically means using Opm::EclipseState instead of the raw deck
for these keywords.

with this, property modifiers like ADD, MULT, COPY and friends are
supported for at least the PERM* keywords. If additional keywords are
required these can be added relatively easily as well.

no ctest regressions have been observed with this patch on my machine.

note that in order to avoid build breakage, this patch needs to be merged synchronously with the respective opm-autodiff and opm-polymer PRs.

also note that the norne deck is broken until OPM/opm-parser#232 is merged. (which should not be too long away.) This _might_ have something to do with this PR but it is more likely that the breakage has already been introduced when EclipseState began to process property modifiers last week. I haven't bothered to check, though....
Thanks for noticing, you're right as usual. Also, since your comment made me a curious, I conferred the RM, and noted that valid decks must at least exhibit the sections RUNSPEC, GRID, PROPS, SOLUTION and SCHEDULE. I've added them to the files which caused test failures (deadfluids.DATA and capillary.DATA) in the latest version of the patch, but I don't care sufficiently to adapt the other data files as well...