Implement three parallel methods for injecting activity into muscle cells

[slarson]

@pgleeson aims to submit a pull request with code works in parallel with the sine wave injected into the muscles, c302 activity injected into the muscles, and the sibernetic_NEURON bridge injecting activity into the muscles and an updated README that explains how you can run each of the three options with a straightforward argument change.

[pgleeson]

This is implemented in the current development branch