Compliant: StructuralAPI: while building a rigid body from a mesh, automatically rotate the rigid dof to be aligned with inertia principal axis (to get a diagonal inertia matrix)

git-svn-id: svn://scm.gforge.inria.fr/svnroot/sofa/trunk@11358 b7f1a160-662a-0410-a6d8-bea165342f16