Fix SBML export species id mismatch with leading underscores (#237)
Fix SBML export species id mismatch with leading underscores (#237)

The SBML exporter makes some assumptions about how PySB names species,
and this changed in the BNG interface code which creates the species
but wasn't updated to match here.
LoLab-VU/pysb,pysb/pysb,jmuhlich/pysb,LoLab-VU/pysb,alubbock/pysb,johnbachman/pysb,johnbachman/pysb