Math: simplified DualQuaternion ScLERP.
Math: simplified DualQuaternion ScLERP.

Making use of sincos() for Dual numbers, constructing DualQuaternion
from dual vector and scalar parts and using
DualQuaternion::isNormalized(). Also updated the math equation to be
consistent with conventions elsewhere.

MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum