EclipseWriter: add possibility to write out the transmissibilities which are used by the simulator

the quantities are attributed to the positive faces of logically
Cartesian cells. I have not found a way to visualize the quanties in
the INIT file using ResInsight, so this is written once per timestep
for now...

Note that the TRANNNC stuff is a bit of a work-arround, but I could not figure out a canonical way to query the grid if a face is a NNC or not. (any hints are appreciated.)