Add Exercise 9.1.
Add Exercise 9.2.
Add potential_energy function to util.

This function follows the algorithm in Section 5.1 of Kane 1985.
Add Example 5.1.
Add Exercise 9.3, update potential energy function.

Add Exercise 9.3 from Kane 1985. Add additional cases where the
potential energy function for a system cannot be found.
Add Exercise 9.4, refactor utility functions.
Add Exercise 9.5.
Add Exercise 9.6.
Add Exercise 9.7.
Fix usage of A_kr in potential energy calculations.

Only use the first p columns in A_kr when calculating potential energy
and generalized active forces from potential energy.
Add Exercise 9.8.
Fix potential energy calculation.

Improve equality checking of partial derivatives.
Add Exercise 9.9.
Add Exercise 9.10.

The exercise as currently worked out does not match answer in text.
Add Exercise 9.11.
Add Exercise 9.12.

WIP. Series expansion of generalized active forces does not complete.
Refactor utility functions.

Remove function_from_partials from potential_energy function. This
allows for independent use, as is desired when calculating dissipation
functions.
Add Exercise 9.13.
Add Exercise 9.14
Fix sorting errors of symbolic expressions.

This is due to changes in symbolic expressions between Sympy 0.7.2 and
Sympy 0.7.3.