[WIP] Reference density state

This is a first idea of how a reference density implementation for different approximations could work.
How about making this more explicit, including a description where the density appears:
```
  This flag determines whether the physical density $\\rho(p,T,\\mathfrak c,\\mathbf x)$ should be used everywhere the density appears in the basic equations \\eqref{eq:stokes-1}--\\eqref{eq:temperature} (if set to true), or if we should use the physical density only the buoyancy term on the right hand side of \\eqref{eq:stokes-1} whereas an adiabatic reference density that only depends on the depth is used in all other places (if the parameters is false).
```
Maybe add a sentence or two which approximations use the latter.