operator * method
Multiply an angle
Implementation
GeoAngle operator *(Object other) {
  if (other is GeoAngle) {
    return GeoAngle(radian: radian * other.radian);
  } else if (other is double) {
    return GeoAngle(radian: radian * other);
  } else if (other is int) {
    return GeoAngle(radian: radian * other);
  } else {
    return this;
  }
}