o simplify EXCH interface:  - change _XYZ_ interface to _3D_ subroutine (with 3rd dim in argument list)  - not often used EXCH S/R (exch_z, exch_uv_agrid): keep only the _3D_ version
o simplify EXCH interface:
 - change _XYZ_ interface to _3D_ subroutine (with 3rd dim in argument list)
 - not often used EXCH S/R (exch_z, exch_uv_agrid): keep only the _3D_ version

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