This function controls velocity(wheel mode speed) of DYNAMIXEL.
  const int DXL_DIR_PIN = 2;
  Dynamixel2Arduino dxl(Serial1, DXL_DIR_PIN);
  dxl.setGoalVelocity(1, 100); //use default raw value
  dxl.setGoalVelocity(2, 10.5, UNIT_RPM); //use RPM
  dxl.setGoalVelocity(3, 15.0, UNIT_PERCENT); //use percentage (-100 ~ 100 %)