Leapfrog integration implemented. 

* Global Time interval now directly used in calculation, turning off manipulation of G and velocities.
* 'R' Keypress reverses timeflow.
* Velocity, position, and acceleration are known at every tick for every particle.
* Calculating all gravitation accelerations is now its own step, making it more straight forward to optimize.  Low-hanging fruit up next includes a factor of 2 speedup.