Add func to Expression to stringify formula correctly
Add func to Expression to stringify formula correctly

This handles both cases where the right-hand-side of the expression is
an algebraic formula (the typical case) as well as a sympy.Symbol
(single Parameter or Observable).

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