suzerain: #3077 delegate for YXZ consistency

This commit should not modify the observable behavior of
Largo.  It is intended to be sure calling preStep_sEta_innery
then preStep_sEta_innerxz is equivalent to preStep_sEta.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@44359 0bd85f0b-c151-43b9-a1e9-27231104551f