suzerain: #3036 adjust convective for y*grdelta

Done by computing an additive factor once per y loop,
and then, effectively, adjusting u.y() -= y_grdelta
for the convective stability checks.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@43551 0bd85f0b-c151-43b9-a1e9-27231104551f