Implement static gravity potential in DerivedGeology.

This is a stepping stone towards implementing gravity support in
ImpesTPFAAD.
Added interface for properties for use with AD code.
Construct (and store) gravity operator on grid.

The gravity flux of phase 'i' across an (internal) interface is then
given by

    gflux = mob_i * (G * rho_i)

assuming 'rho_i' contains the phase density (at reservoir conditions)
of phase 'i' and 'mob_i' is the phase mobility of phase 'i' at the
interface.
Add tentative implementation of gravity flux.
Merge remote-tracking branch 'atgeirr/master'