PVT hierarchy chart documentation

[ver2king]

I refer to the following:

https://geosx-geosx.readthedocs-hosted.com/en/latest/coreComponents/constitutive/PVTPackage/docs/main.html

The hierarchy chart and its included descriptions are not clear to me, and I could not see any documentation regarding customization for a new fluid model. Please correct me in case such customization does not require any further customization within the PVT package (and should be within some other GGEOSX dependencies)