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
depends on coordinate system. I assumed that the grid goes into negative Y direction. (like for the Z axis.) Anyway, comment adapted.