implement support for the NTG, MULTPV, and MULT[XYZ]-? keywords

or rather grid properties, since they can be edited using MULTIPLY and
friends. note that the property interfaces must be extended for this
because EclipseState is not passed to the DerivedGeology class and is
not even available in some cases...

this PR depends on the OPM/opm-core#610
Nit: `faceTag==2` is actually *back* although it certainly is `YMinus`.  Similarly `faceTag==3` is front (and `YPlus`).