implement pore volume and transmissibility 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.)
add a unit test for the implementation of transmissibilty multipliers in DerivedGeology

for the legacy C-style grid the unit test is more or less complete (it
does not test FAULTMULT and NNC, etc, but these could be added with
sufficient determination), for Dune::CpGrid it currently does not
really check anything because I have not found a good way for CpGrid
to produce the "global" intersection index of an intersection...