Add Exercise 3.10.
Finish Exercise 10.3.
Fix vc_matrix calculation, Exercise 9.8.

Calculation of velocity constraint matrix finds u_s in terms of
u_r (r=1,...,p). u_s should not be an instance of Derivative not should
it have derivative variable (t,).
Fix Exercise 9.8 to print out equalities equivalent to solution in Kane
text and remove erroneous exercise number.
Add functions to util to find inertia coefficients.
Add Exercise 10.12.