suzerain: adjust CFL for gr_{X_s} in models

Per the spatiotemporal draft-in-progress, the convective
modifications for the spatiotemporal class of models use
gr_{X_s} instead of gr_{T_s}.  So adjust the dynamic
timestep computation to translate from one to the other.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@44912 0bd85f0b-c151-43b9-a1e9-27231104551f