HOTFIX: Homo-Dimer Reaction Propensities

[BryanRumsey]

Case 1

This case is impossible to catch since python dictionaries don't support duplicate keys.

r1 = spatialpy.Reaction(
    name="r1", reactants={"s1":1, "s1":1}, products={"s1":1, "s2":1}, rate=0.1
)

Case 2

S1 = spatialpy.Species(name="s1", diffusion_coefficient=0.1)
r2 = spatialpy.Reaction(
    name="r2", reactants={"s1":1, S1:1}, products={"s1":1, "s2":1}, rate=0.1
)

Propensity Bug

r2
	Reactants
		s1: 1
	Products
		s1: 1
		s2: 1
	Propensity Function: (0.1*s1)

Fixed Propensity

r2
	Reactants
		s1: 2
	Products
		s1: 1
		s2: 1
	Propensity Function: (((0.1*s1)*(s1-1))/vol)

Case 3

r3 = spatialpy.Reaction(
    name="r3", reactants={"s1":1}, products={"s1":1, "s2":1}, rate=0.1
)
r3.add_reactant("s1", 1)

Propensity Bug

r3
	Reactants
		s1: 1
	Products
		s1: 1
		s2: 1
	Propensity Function: (0.1*s1)

Fixed Propensity

r3
	Reactants
		s1: 2
	Products
		s1: 1
		s2: 1
	Propensity Function: (((0.1*s1)*(s1-1))/vol)

closes #328