Export expressions as SBML assignment rules (#372)
Export expressions as SBML assignment rules (#372)

* encode expressions as assignment rules

* fix expansion of expressions in sbml export of initial conditions and rates

* export sbml reaction names according to the rules they implement and dont expand the expression in exported sbml InitialAssignment
pysb/pysb,LoLab-VU/pysb,LoLab-VU/pysb,johnbachman/pysb,alubbock/pysb,jmuhlich/pysb,johnbachman/pysb