microtubule simulation: tension-dep. rescue and length-dep catastrophe implem.

A few new functions were made to implement rescue and catastrophe probabilities the way they were done in my other papers. If you run spindle_simulation, these processes function on a single microtubule attached to a spring attached to a fixed wall. It should be easy to extend to two attached microtubules.

Also, under the main simulation part, there is a short piece of code in spindle_simulation that lets you see how different rescue and catastrophe coefficients affect the per-second chances of these events over multiple microtubule lengths and tensions.

To do:

add length-dependent rescue, tension-dependent catastrophe
make length- and tension-dependent processes swappable
add more plotting code to:
   show combined effect of tension and length on rescue / catastrophe
   show frequencies of rescues / catastrophes that actually occur when MTs are connected in a spindle