suzerain: improve isentropic handling

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@42735 0bd85f0b-c151-43b9-a1e9-27231104551f
suzerain: push a, a_xi, a_y into primitive

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@42736 0bd85f0b-c151-43b9-a1e9-27231104551f