simple two-microtubule, one hookean spring model

This update brings a new spindle-associated structure, the hooke_spring, which holds the data on a hookean spring and its history. Next, a few functions were created to model the spring. Next, a few functions were created to track information between microtubules and a hooke_spring. Finally, the model was re-written in spindle_simulation (the top-level script) to run a single chromosome / two kMT model.