Changed from storing positions at two adjacent time steps to storing position and velocity at one time step.
Changed from storing positions at two adjacent time steps to storing
position and velocity at one time step.

Commented the code much more thouroughly, so there is a chance someone
else may actually be able to understand it.

Still TODO: Read all configuration parameters from an input file. The
output is especially hackish, and should be addressed next.

fcahoon/physmod