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.

Updated for recent changes to CLUBB.