Function add

add(a, b, dst?): Quat
Adds two quaternions; assumes a and b have the same dimension.
Parameters
- a: Quat
  Operand quaternion.
- b: Quat
  Operand quaternion.
- Optional dst: Quat
  quaternion to hold result. If not passed in a new one is created.
Returns Quat
A quaternion that is the sum of a and b.