suzerain: expose p, p_y, p_xy for Largo

We'll need these details for Largo baseflow computations,
so make them available here.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@42556 0bd85f0b-c151-43b9-a1e9-27231104551f
suzerain: extract some reused values

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@42557 0bd85f0b-c151-43b9-a1e9-27231104551f