Tidy if/else logic for generating reactions_bidirectional
Tidy if/else logic for generating reactions_bidirectional

Thanks to @jmuhlich for the suggestion

jmuhlich/pysb,johnbachman/pysb,LoLab-VU/pysb,johnbachman/pysb,pysb/pysb,alubbock/pysb,LoLab-VU/pysb