suzerain: #3010 fsolver in baseflow_compute_thicknesses

These look to be the right APIs.  On to making them public
and then testing.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@42927 0bd85f0b-c151-43b9-a1e9-27231104551f