Function slerp

slerp(a, b, t, dst?): Quat
Spherically linear interpolate between two quaternions
Parameters
- a: Quat
  starting value
- b: Quat
  ending value
- t: number
  value where 0 = a and 1 = b
- Optional dst: Quat
  quaternion to hold result. If not passed in a new one is created.
Returns Quat
A quaternion that is the result of a * b