Crn

A few critical bug fixes:
- `0 >> r` would be interpreted as `Reaction(Species('r'), Species('nothing')`, instead of the correct `Reaction(Species('nothing'), Species('r')`. That is, the `0` was always a product when used in a reaction.
- `r + r >> b` would be interpreted as `r >> b`, since species are keys in a `dict`.

Installation Simplification
The StochPy fix necessary to run the stochastic simulation portion of the package now runs automatically when importing `crn`.

Bug Fixes
Importing `stochpy` without installing it would quit and not show any error. Now a normal ImportError is raised and shown as expected.

Features
- Clean & concise reaction literal syntax
- Deterministic & Stochastic simulations
- Deterministic simulations use initial concentrations
- Stochastic simulations are supported thanks to [StochPy](https://github.com/SystemsBioinformatics/stochpy), but we only make use of their Direct Solver (aka Gillespie's Method).
- Quick plotting of all species throughout the simulation
- Easy access to each species' time-series
- Function validation using deterministic simulation
- Given a CRN and a function, we can check if the CRN actually computes the provided function (with an arbitrarily small chance of error)

Future Plans
- More fine-tuned/advanced plots
- Specify which species are relevant
- Contour plots
- Add a test suite
- Account for different volumes in stochastic simulations
- Add a simple logic gate submodule for quick composition of analog/discrete logic gates.
- Likely other things I'm forgetting