Prusa XL and MK4 Rocket Engine (edited)

I saw that the original Rocke Engine for the XL has a significant issue that keeps coming up when you slice it.  It shows as a model error.  Turns out to be a ledge that it looks like two models were joined.  I just removed the insides of the model and added a more simplified curve that would be more indicative for a rocket engine.  While I was at it I decided to provide a split model that is more easily printed for those without an XL.  

I also edited the MK4 version, the whole model was reduced by about 10% and the rocket bell was significantly shorten.  This version is scaled to be the same as the XL and can be interchanged with either bell.