Added support for net-to-gross

[francoishamon]

This PR adds the support of the old-fashioned net-to-gross property found in many reservoir models (multiplier for the porosity and the horizontal permeability). Although this could also be done externally, I thought it would be useful for users who may not know how net-to-gross works to have this inside GEOSX.

In passing, the PR also adds the PorosityExtrinsicData (not used yet, but added by Ryan for stabilisation and by me for norm calculation in other open PRs).

Requires rebaseline.