suzerain: #3004 draft neg_Bq functionality

This gets neg_Bq into bl.dat under bl.wall output line.
Appears to be giving sensible results for manual.gold cases,
but I must admit lingering doubts as to the u_tau scaling
in this context.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@42822 0bd85f0b-c151-43b9-a1e9-27231104551f