add support for the NTG keyword

since NTG modifies PERMX and PERMY but not PERMZ, and because PERM[YZ]
default to PERMX, the permeabilities on the main diagonal are now
always copied so that tampering with them does not lead to a gazillion
of cases...

also, I've made the `assign*()` methods of the rock properties public and renamed them to `update\1()` so that they can be more used if the schedule decide that an update of these properties is required...