Return the quaternion which aligns XB with XA
Return the matrix which aligns structure XB to be as similar as possible to structure XA. To be precise, rotate XB, so as to minimize the distance |XA - XB|.
Rotations will be done around the origin, not the center of mass
Rotational alignment follows the prescription of Kearsley, Acta Cryst. A, 45, 208-210, 1989 http://dx.doi.org/10.1107/S0108767388010128