Changes

This commit contains code changes to implement modifications on limiters in three places:

1. remove the limiters in denominator of equation for
   brunt_vaisala_freq_sqd_smth, which affects the computed
   eddy dissipation time scale in turbulent fluxes (wpxp).
   (in mixing_length.F90)

2. reduce the threshold values of limiters in the equation
   for richardson number (sqrt_Ri_zm)
   (in mixing_length.F90)

3. introduce the smoothed max/min function for limiters in
   equation of Cx_fnc_Richardson.
   (in advance_helper_module.F90).

After the modification, we also apply a zt2m(zm2zt) smoothing
on the calculated quantities. These modifications are found to
be benificial for improving solution convergence in CLUBB-SCM

The code changes are controlled by a newly introduced flag named
"l_modify_limiters_for_cnvg_test", which is set to .false. (meaning that
the modificaitons on limiters is turned off) by default.

Originally committed by Shixuan.

This commit contains code changes to implement modifications on limiters in three places:

1. remove the limiters in denominator of equation for
   brunt_vaisala_freq_sqd_smth, which affects the computed
   eddy dissipation time scale in turbulent fluxes (wpxp).
   (in mixing_length.F90)

2. reduce the threshold values of limiters in the equation
   for richardson number (sqrt_Ri_zm)
   (in mixing_length.F90)

3. introduce the smoothed max/min function for limiters in
   equation of Cx_fnc_Richardson.
   (in advance_helper_module.F90).

After the modification, we also apply a zt2m(zm2zt) smoothing
on the calculated quantities. These modifications are found to
be benificial for improving solution convergence in CLUBB-SCM

The code changes are controlled by a newly introduced flag named
"l_modify_limiters_for_cnvg_test", which is set to .false. (meaning that
the modificaitons on limiters is turned off) by default.

Originally committed by Shixuan.

Updated in response to recent changes to CLUBB.