WIP Clarify z is positive up and eliminate -A in gravprisms

[PaulWessel]

See #6818 for background. This PR improves the discussion of water depth (related to density contrasts) and reference depth (-Z, now specified as a distance). Also added an include _rst file that explains the geometry. This file will be included in other modules as I revise them.

Work in progress so no need to review yet - I am still working on it