suzerain: #2989 static inline version of omsect()

See https://github.com/RhysU/intersection/releases/tag/0.1.0
for the origin of the omsect function including its derivation.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@42414 0bd85f0b-c151-43b9-a1e9-27231104551f
suzerain: #2989 integration over part of domain

Adds omsect()-based logic into bspline.c
Updates C API and C units tests.
Updates C++ providing backwards-compatible default arguments.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@42415 0bd85f0b-c151-43b9-a1e9-27231104551f