Bodner diag: Fix ustar and dims

Fix two issues which emerged from the Bodner length scale diagnostic
after the cuberoot ANSWER_DATE was introduced.

The split of the loop due to the cuberoot caused ustar to be assigned in
a separate loop, leaving it uninitialized for the length scale
diagnostics loop.  It also had its dimensions removed.

However, the whole point of the new cuberoot() function was to preserve
its dimensions, so there was no need for the intermediate dimensional
manipulation, and all dimensionality should be shifted to the
registration.

This patch removes the intermediate dimensions and also explicitly
defines it as L2/Z rather than L.

Having said all that... I am uncertain about the dimensions here.  Ustar
is tagged as H/T in some places and Z/T in others.  I think this needs
to be reviewed.

src/parameterizations/lateral/MOM_mixed_layer_restrat.F90

@@ -979,16 +979,18 @@ subroutine mixedlayer_restrat_Bodner(CS, G, GV, US, h, uhtr, vhtr, tv, forces, d
     if (CS%id_lfbod > 0) then
       ! Calculate front length used in B22 formula (eq 24).
 
+      w_star3 = max(0., -bflux(i,j)) * BLD(i,j)
+      u_star3 = U_star_2d(i,j)**3
+
       ! Include an absurdly_small_freq2 to prevent division by zero.
       f2_h = max( &
           (0.25*((G%CoriolisBu(I,J)  + G%CoriolisBu(I-1,J-1)) &
             + (G%CoriolisBu(I-1,J) + G%CoriolisBu(I,J-1))))**2, &
           absurdly_small_freq2)
 
       lf_bodner_diag(i,j) = &
-          0.25 * (cuberoot(CS%mstar * u_star3 + CS%nstar * w_star3)**2 &
-            * (US%m_to_L * GV%m_to_H * US%T_to_s**2) &
-          ) / (f2_h * max(little_h(i,j), GV%Angstrom_H))
+          0.25 * cuberoot(CS%mstar * u_star3 + CS%nstar * w_star3)**2 &
+            / (f2_h * max(little_h(i,j), GV%Angstrom_H))
     endif
   enddo ; enddo
 
@@ -1809,7 +1811,7 @@ logical function mixedlayer_restrat_init(Time, G, GV, US, param_file, diag, CS,
         'm2 s-3', conversion=(US%Z_to_m**2*US%s_to_T**3))
     CS%id_lfbod = register_diag_field('ocean_model', 'lf_bodner', diag%axesT1, Time, &
         'Front length in Bodner mixed layer restratificiation parameterization', &
-        'm', conversion=(US%L_to_m))
+        'm', conversion=(US%L_to_m**2)*US%m_to_Z)
   endif
 
   ! If MLD_filtered is being used, we need to update halo regions after a restart