handle bng reverse reactions more consistently
handle bng reverse reactions more consistently

* if bng emits the (reverse) version of a reaction before the forward one, we
    now represent that correctly in reactions_bidirectional

jmuhlich/pysb,pysb/pysb,alubbock/pysb-legacy,alubbock/pysb-legacy,alubbock/pysb-legacy,neurord/pysb,spgarbet/pysb,alubbock/pysb-legacy,spgarbet/pysb,johnbachman/pysb,neurord/pysb,johnbachman/pysb,d-fan/pysb,d-fan/pysb,LoLab-VU/pysb,d-fan/pysb,LoLab-VU/pysb,neurord/pysb,spgarbet/pysb,alubbock/pysb