suzerain: gather gamma, compute other postprocessed quantities in summary

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@45226 0bd85f0b-c151-43b9-a1e9-27231104551f
suzerain: #3111 extract logic from driver.cpp

This isn't tricky, but its lengthy enough that I don't
want to repeat it over-and-over.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@45227 0bd85f0b-c151-43b9-a1e9-27231104551f