Nonphysical inertia tensors in the example notebooks

[rdeits]

I noticed while playing around with the updates to MeshCatMechanisms that the spatial inertias in all of the simple examples are set to something like I * axis * axis'. That gives the right results in the plane, but it's not actually a physically possible distribution of mass because it violates the triangle inequality (\lambda_i + \lambda_j \geq \lambda_k for any ordering of the eigenvalues \lambda). The correct tensor for a rod aligned along the x axis would be I * SDiagonal(0, 1, 1) (or maybe SDiagonal(0.01, 1, 1) if we want a slightly finite rod).

Do you think it's worth fixing the examples?