Fix sorting of f args in PE function.

Fix sorting of arguments of f functions when calculating a potential
energy function of a system.
Do not modify u in util functions.

Do not modify list of independent generalized speeds u in functions
potential_energy, generalized_active_forces_V,
generalized_inertia_forces_K. Minor code refactor.
Add Fr function to util.

Add a function to find the generalized active forces, Fr, of a system
from the system's kinetic energy.
Add Lagrange's equations function to util.

Add a function to find Lagrange's equations of the second kind for a
system.
Add Exercises 11.12, 11.13.