Fix typo.
Fix methods using inertia in RigidBody.

Change methods angular_momentum, kinetic_energy to use the central
inertia dyadic instead of the user supplied inertia dyadic. The body's
central inertia dyadic is now calculated and stored when the inertia is
set.
Add test for RigidBody central_inertia property.
Add additional test for KanesMethod.

Add an additional test for KanesMethod that checks if the calculated
generalized inertia forces are equivalent whether the inertia for rigid
bodies is set about the mass center or an arbitraty point.
Merge pull request #2194 from oliverlee/rbfix

Fix methods using inertia in RigidBody