Trans mult container

This PR creates a container to hold on to the Transmissibility multipliers and adds one such instance to the EclipseState class.The plan is that the combined effect of all the MULT(X|Y|Z|FLT|REGT) keywords should be applied in here.

Good if the opm-core consumers would comment on the API.
I think the doc for the method should be placed with the method (the first of them, the rest can point to that).

I think it is a little unclear what is meant: what is the allowed input range? Is it different for the different functions?

I think an alternative API may be clearer:
   double getFaceMult(const int i, const int j, const int k, CartesianFace f);
where the (i,j,k) refers to a cell, and CartesianFace is an enum a la { XMin, XMax, YMin, YMax, ZMin, ZMax }.