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...