Increase bftt_dz_sum_thres

[mnlevy1981]

When testing the MOM6 MARBL driver with a 75 level hybrid vertical coordinate, I was exceeding bftt_dz_sum_thres in several columns. The current default is bftt_dz_sum_thres = 1e-14, and I have been changing it to bftt_dz_sum_thres = 1e-12 for my runs -- I think it's reasonable to change the default threshold by two orders of magnitude, as seen in the MOM6 example the double precision arithmetic can introduce numerical errors larger than the current 1e-14 threshold.