Interpolate quaternions
QI = qinterp(Q1,Q2, S)
QI = qinterp(Q1,Q2, S)
is a unit-quaternion that
interpolates a rotation between Q1 for S=0 and Q2 for S=1.
If S is a vector QI is a list of quaternions, each element corresponding to sequential elements of S.
This is a spherical linear interpolation (slerp) that can be interpretted as interpolation along a great circle arc on a sphere.
The value of S is clipped to the interval 0 to 1.