fromArray method
array of length 3 or 4. The optional 4th argument corresponds
to the order.
Assigns this euler's x angle to array[0].
Assigns this euler's y angle to array[1].
Assigns this euler's z angle to array[2].
Optionally assigns this euler's order to array[3].
Implementation
Euler fromArray(List<double> array) {
  _x = array[0];
  _y = array[1];
  _z = array[2];
  if (array.length > 3) _order = RotationOrders.values[array[3].toInt()];
  onChangeCallback();
  return this;
}