grid properties: add support for the NTG keyword

Whoever came up with that keyword deserves a spanking because its
semantics are adapting thickness of the grid layers using the rock
properties without modifying the grid (and thus the output is not
showing what is actually used). As I understood that whole affair,
this is also done incorrectly, because the flow is not distorted in
depth direction but this the permeability should be divided by the NTG
values as the fluids seep through the thinner layers more quickly...

Anyway, we have to implement it, so here we go.
Merge pull request #245 from andlaus/add_ntg_gridproperty

grid properties: add support for the NTG keyword