Implement pore volume and permeability multipliers

this means that the NTG, MULTPV and MULT[XYZ]-? keywords are now
supported.
    
Actually FAULTS and MULTFAULT are supported too, but that's abstracted
away by opm-parser's TransMult class. (Kudos to [at]joakim-hove for
implementing this.)

this PR is the successor of #161. All important design decisions have been discussed there already (in particular that  `DerivedGeology` should be instantiated externally from the simulator and passing it the `EclipseState` object instead of extending the properties interface which implies the removal of the non-deck code paths), but I think that the discussion of #161 is getting too long and would now also be a bit off-topic, so I start a new PR...