suzerain: #2503 #2991 baseflow freestream contraints

Does two futzy things:
 * Ensures a non-trivial baseflow is present when Largo requires it.
 * Matches mean freestream constraints to the baseflow when present.
 * Corrects save/load operations when !sg->baseflow.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@42466 0bd85f0b-c151-43b9-a1e9-27231104551f