Changes

While it doesn't affect results, I fixed what appears to be an
obvious bug in advance_xm_wpxp. If everything is at a singular zm
level, the referenced height shouldn't be gr%zt, but rather gr%zm.

I am updating the loop in subroutine diagnose_xpwp so that it doesn't
loop over the model upper or lower boundaries. Calculations at those
levels are irrelevant to the results, and their inclusion causes
bitness differences between the master and clubb_ghost_exorcism
branches. Since this code change causes differences in the zm
stats file at the upper and lower boundaries for a few fields, this
commit is technically BIT_CHANGING, although it should be noted
that it does not change the simulation results.