BUG: Fix the definition for PPG.

The `ppg` unit is defined as a density "pounds-mass per US gallon". It is often used to measure the density of mud used in wells to balance the pressure outside of the well. It is mostly used in a hydrostatic pressure calculation `P = density*g*depth` where `g` is the acceleration due to gravity. The previous definition of `ppg` in this package folded in this constant into the unit to give the illusion that this was a pressure gradient unit. This is inconsistent with the other units used in the `mud_weight` family of units, which are also mass-density and not pressure gradients.

This PR fixes the definition of `ppg` and gives guidance on converting old code in the changelog. This is unlikely to negatively affect active projects that I am aware of.