+Clean up dimensional rescaling in write_u_accel

  Pass wind stresses to write_u_accel() and write_v_accel() in [R L Z T-2 ~> Pa]
without requiring external rescaling first.  Also cleaned up the dimensional
scaling in MOM_PointAccel.F90, so that all arguments and internal variables use
the dimensionally rescaled variables with cleaner and standardized syntax, and
there are comments clearly describing all real variables in this module.
Moreover, several rescaling bugs were corrected in these routines, so that if
there are velocity truncations, now the output will be invariant to rescaling,
whereas before some reported values were not being properly scaled back to MKS
units.  All answers and output from the MOM6-examples regression suite are
bitwise identical.