Threshold pressure function

This adds a new utility function that looks in the deck and eclipseState to extract threshold pressures. They are returned in the form of a vector of thresholds, one per grid face.

It may be a good idea to move some of this logic into EclipseState, but for now I think it is good enough to have it in a free function as done here.

This depends on OPM/opm-parser#292.