suzerain: #3099 baseflow + 3D = no driving

See comments inline for rationale.  Long-suggested by Victor
and an outcome of work on #3088 (especially update 17).

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@44713 0bd85f0b-c151-43b9-a1e9-27231104551f