suzerain: fix bar_pairs

Recent fixes to the SpectralData, etc for pickling broke
the bar_pairs functionality.  This should restore it to
its previously working form and checks out as sane
against my Coleman data vs Coleman JFM 1995.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@45523 0bd85f0b-c151-43b9-a1e9-27231104551f