Problematic results for a simple reversible autocatalytic reaction

[BryanRumsey]

This issue was originally reported here

Fix

• Remove the 0.5 term when creating homo-bimolecular propensities
• Update unit tests

[PENG-Zhen]

I guess I know a reason behind the bug: simply a notation issue.
The "k" value and "c" value are sometimes mistaken. Usually people use "k" to denote the "reaction rate constant" under mass action kinetics, while in GA, "c" is usually used to denote the "stochastic reaction constant". The original error was probably because I entered the "k" value as the tutorial on your website used the symbol "k", but the program actually considered that it was the "c" value by default. And I also messed up when I stated the error on github; I should have said "reaction rate constant" instead of "stochastic reaction constant".
I guess it might be a good idea to clearly state in a tutorial or documentation about whether a "k" value or a "c" value should be used, and the trickiness when the propensity of a reaction requiring >=2 same reactants to occur (e.g., for 2X=>Y, propensity = c * 0.5 * x * (x-1), c = 2 * k / volume).