Implements Thomas Toniazzo's bug fix for spike removal

in the monotonic flux limiter.  See

https://github.com/larson-group/clubb_release/pull/11/files

This code change is
expected to be BIT_CHANGING for cases in which
`l_predict_upwp_vpwp = T`, `l_mono_flux_lim_um = T`
or `l_mono_flux_lim_vm = T`, and the monotonic
flux limiter is triggered.

This bug fix prevents non-conservation of momentum
when the vertical integral of either of the wind components `um` or `vm` is negative.

src/CLUBB_core/mono_flux_limiter.F90

@@ -856,7 +856,7 @@ subroutine monotonic_turbulent_flux_limit( nz, ngrdcol, gr, solve_type, dt, xm_o
 
             !Check to ensure the vertical integral is not zero to avoid a divide
             !by zero error
-            if (xm_vert_integral < eps) then
+            if ( abs(xm_vert_integral) < eps ) then
                write(fstderr,*) "Vertical integral of xm is zero;", & 
                                 "mfl will remove spike at top of domain,", &
                                 "but it will not conserve xm."