suzerain: improve const-ness of timeprefix

Marking this const API as such.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@42879 0bd85f0b-c151-43b9-a1e9-27231104551f
suzerain: extract log_quantities_local_helper

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@42880 0bd85f0b-c151-43b9-a1e9-27231104551f