Reaction network

[justinlaughlin]

Implement ability to generate a reaction network from a model file.

Pysb and NetworkX are two packages to check out.

[Rabona17]

Benefits:
Allows user to check the reaction process by presenting a visualized reaction graph. Users will be able to check the value of fluxes manually.

Use cases:
When the user wants to visualize the reaction process.

Sub issues:
1.Needs to find a good way to assemble fluxes that happened on only one side of the compartment
2.Testing required