/// <summary>
 /// Remove a previously added interaction
 /// </summary>
 /// <param name="i">interaction to remove</param>
 public void RemoveInteraction(Interaction i)
 {
     interactions.Remove(i);
 }
 /// <summary>
 /// Adds a specific interaction to the PlotSurface2D
 /// </summary>
 /// <param name="i">the interaction to add.</param>
 public void AddInteraction(Interaction i)
 {
     interactions.Add(i);
 }