Add parameter to SkeletonDynamics::computeDynamics to allow the user to disable the automatic calculation of the inverse of the mass matrix.

The inverse of the the mass matrix is for example used by the contact handling code. But if the user only wants to simulate the dynamics of a skeleton without considering contacts, the calculation of the inverse is wasting time.
More clean-up and small speed improvements
When calling Skeleton::setPose() we do not need to calculate the Jacobians since that is also done by SkeletonDynamics::computeDynamics()