Fix/oduran/consistent compositional buoyant flux

[OmarDuran]

This PR deals with refactoring the buoyant flux part in the compositional MFD component. This PR depends on PR https://github.com/GEOS-DEV/GEOS/pull/3765

To achieve a consistent implementation of gravity fluxes two concepts are introduced, the trace of the quantity: (rho_delta - rho_gamma) K grad (z) which is the driving buoyant flux. the trace of the full buoyant flux associated with the pair phase delta - gamma. the traces of total mass mobility \Lambda and the phase mobilities lambda_delta and lambda_gamma are defined by the upwind scheme in question, in this case Hybrid upwinding.