Model updated to allow for a fixed kinetochore length

The kinetochore has a non-zero length, probably about 68nm long, according to Haase et al. 2013 Curr Biol. This is important because right now we are treating the plus ends of the microtubules as where the pericentromere starts, when there is actually about 68nm space between these things. This is important because if kMT dynamics depend on tension, then the tension is actually less for a given kMT separation distance than we previously assumed, because the pericentromere spring is stretched kinetochore_length * 2 less.

To fix this, spindle_simulation takes in another constant, kinetochore_length. This is fed through to runModel and determineStretchLength, which both now make the pericentromere spring length 2*kinetochore_length less than the distance between the kMTs in the spindle.