property objects: export the NTG, MULTPV and MULT[XYZ]-? fields

These are required to calculate the transmissibilities. Since the
class which mingles these objects together only gets a property object
but not the EclipseState (which might be not available after all),
these quantities must be included in the property interfaces. Adapting
this boiler-plate code is the vast majority of this patch.

Note that all of these arrays store one value per logically cartesian
cell as the nice file format says that the transmissibility multiplier
for the X face of cell i is the product of MULTX and MULTX- for the
next cell. From the answers I got so far, I infer that this is the
case even if the next cell is inactive, so there must be a way to
access that information from the place where the transmissibility is
calculated.

This PR is a prerequisite to implement these modifiers in opm-autodiff.