added comparision to built in epsilon makros and exemplified a not so trivial division error!
finished todo list

- corrected analytical based start position. Added correction to
  finite start position.
- Added output of start angle of asymptote for analytical seeding,
  which is the reason for the error on the scattering angle and can
  therefore be used to approximate that error
- implemented verlet correct now. Energy is constant for dt->0

still todo:

- center of mass system
- relativistic kinematics