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.

This commit is being made to keep the stats zt output from the
master branch inline with the output from the clubb_ghost_exorcism
branch. In short, the only change is that the value of
a couple of inputs relative to wpxp turbulent advection from
thermodynamic level 1 are set to 0 for stats output in order to
keep the master and ghost exorcism branches bit-for-bit with
each other. The output at those levels are not used in
any calculations and are unnecessary. However, this change
is technically BIT_CHANGING.