o store 3-D (in-situ) density in commom block (DYNVARS.h)  - save 1 rho computation.  - no longer modify T & S inland when using down_slope pkg.
o store 3-D (in-situ) density in commom block (DYNVARS.h)
 - save 1 rho computation.
 - no longer modify T & S inland when using down_slope pkg.

altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h