IMPORTANT: Option VAR_RHO_2D in the TLM, RPM, and ADM

[arango]

The option VAR_RHO_2D activates the baroclinic pressure gradient response to the perturbation of the free surface in the presence of stratification and bathymetry. This option does not pass the sanity check of the tangent linear (TLM) and adjoint (ADM) models. Thus, in globaldefs.h we had the constraint,

#ifdef SOLVE3D
# if !(defined ADJOINT || defined TANGENT)
#   define VAR_RHO_2D
# endif
#endif

which also affects the nonlinear kernel. This constraint is removed, and we use VAR_RHO_2D_NOT_YET in the TLM, RPM, and ADM kernels instead. Removing this term can be considered part of the linearization.