Bug fix to compute Rd_dx whenever MEKE is used

The default config of MEKE uses Rd_dx to compute barotropic and bottom energy fractions.
 If Rd_dx not computed here, it will be set to zero in MEKE and as a result gamma_b=gamma_t=1.

src/parameterizations/lateral/MOM_lateral_mixing_coeffs.F90

@@ -804,6 +804,7 @@ subroutine VarMix_init(Time, G, param_file, diag, CS)
   CS%calculate_cg1 = CS%calculate_cg1 .or. use_FGNV_streamfn
   call get_param(param_file, mdl, "USE_MEKE", use_MEKE, &
                  default=.false., do_not_log=.true.)
+  CS%calculate_Rd_dx = CS%calculate_Rd_dx .or. use_MEKE               
   CS%calculate_Eady_growth_rate = CS%calculate_Eady_growth_rate .or. use_MEKE
   call get_param(param_file, mdl, "KHTR_PASSIVITY_COEFF", KhTr_passivity_coeff, &
                  default=0., do_not_log=.true.)