In the PDB representation of the frame order motion for the rotor and isotropic cone models (rotor,
free rotor, isotropic cone, free rotor isotropic cone, and torsionless isotropic cone), the X and Z
axes were swapped. This is because the eigenframe of the motion was being incorrectly constructed
via the lib.geometry.rotations.two_vect_to_R() function.
For better control, the specific_analyses.frame_order.geometric.frame_from_axis() function has been
created. This constructs a full motional eigenframe from the Z-axis. The problem was detected via
the new Frame_order.test_pdb_model_rotor system test.
