Zero stratospheric humidity increments

@jianjunj found that we do not yet have in JEDI the zeroing of the stratospheric humidity increments like added to GSI.

We should add this to the increment post processing code for FV3-JEDI. See https://github.com/NOAA-EMC/GSI/blob/dcf055d690ecd132500f03a978b308b98edf375e/src/gsi/write_incr.f90#L551-L595