Fix calculation of constraint forces in hw8

In homework 8.4, find equations of motion of the cart using 3
generalized coordinates and a non-holonomic constraint in hw8.4.py.
Using the closed-form solution of lagrange multiplier lambda, fix
calculation of constraint forces on the cart axle in the x and y
directions in simulate_cart.m.
Update axle constraint force figure for hw8 solution
Finish solution for hw8.4

Add calculation of constrain forces. Add changed code in simulate_cart.m
and cart_equationsofmotion.m.