function setAxisAngle(out, axis, rad) {
    rad = rad * 0.5;
    var s = Math.sin(rad);
    out[0] = s * axis[0];
    out[1] = s * axis[1];
    out[2] = s * axis[2];
    out[3] = Math.cos(rad);
    return out;
  }
Sets a quat from the given angle and rotation axis,
then returns it.

@param {quat} out the receiving quaternion
@param {ReadonlyVec3} axis the axis around which to rotate
@param {Number} rad the angle in radians
@returns {quat} out