Add method to clear sim targets (#733)

Closes #731

photon-lib/src/main/native/include/photonlib/SimVisionSystem.h

@@ -108,6 +108,14 @@ class SimVisionSystem {
         ->SetPose(target.targetPose.ToPose2d());
   }
 
+  /**
+   * Clears all sim vision targets.
+   * This is useful for switching alliances and needing to repopulate the sim
+   * targets. NOTE: Old targets will still show on the Field2d unless
+   * overwritten by new targets with the same ID
+   */
+  void ClearVisionTargets() { targetList.clear(); }
+
   /**
    * Adjust the camera position relative to the robot. Use this if your camera
    * is on a gimbal or turret or some other mobile platform.