Lesson 3 completely rewritten for correct full model, code needs work

I have re-worked the formulation of the full model, with physically relevant parameters remaining in the equations and without any of that funky business of making g=1. The system looks more messy, but the variables have the correct physical interpretation: velocity is velocity, and the damping is inversely proportional to aerodynamic efficiency, which is important physically.
However, I have not revised the code to reflect the new equations. Let's continue working in this pull request until that is done.
Someone please go over my equations and do a sanity check.