Sobol' quasi-random points for the numerical integration are now generated separately for both
torsion angles, and two separate sets of rotation matrices for both angles for each Sobol' point are
now pre-calculated in the create_sobol_data() target function method. The calc_vectors() target
function method has also been modified as the lanthanide to pivot vector is to the second pivot in
the double rotor model rather than the first. The target function itself has been fixed as the two
pivots were mixed up - the 2nd pivot is optimised and the inter-pivot distance along the z-axis
gives the position of the 1st pivot.
For the lib.frame_order.double_rotor module, the second set of Sobol' point rotation matrices
corresponding to sigma2, the rotation about the second pivot, is now passed into the
pcs_numeric_int_double_rotor() function. These rotations are frame shifted into the eigenframe of
the motion, and then correctly passed into pcs_pivot_motion_double_rotor(). The elimination of
Sobol' points outside of the distribution has been fixed in the base pcs_numeric_int_double_rotor()
function and now both torsion angles are being checked.
