Add Documentation for D'Orsogna Model

[Kyle Williams]

The following need documentation

• class Dorsogna(MotionModel)
• def reflect(self, old_pt, new_pt, index) -> tuple:
• def gradient(self, xs, ys):
• def time_derivative(self, _, state):
• def update_points(self, old_points, dt) -> list:

The format for the documentation should be

## One line description .
# More detailed information about the class or function.
#  etc.

Good documentation describes what a function does, the problem it solves, as well as any external information that isn't visible or obvious in the code itself about how the problem is solved. Avoid simply rewriting the code.