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.
Merge pull request #598 from andlaus/enable_property_modifiers

Glue in support for the grid property modifier keywords