Python port of Matlab vclamp simulation, and expanded NEURON code

This is a port of the vclamp.m part of the Boyle & Cohen (2008) Matlab simulation to Python.

The changes to neuron_implementation/main.py are a separate first attempt at modelling a muscle cell with muscle arms. I'm not sure if there's a way to split this into a separate pull request, even though it's a different piece of work.