title: Changed in XCode to mapping model aren't updated in simulator bundle
body: <p>I created a mapping model to migrate between two core data models. I then edited my mapping model, (added a custom migration policy, changed a mapping name, etc.), ran the simulator, and... nothing changed. The old mapping model was loaded, ignoring my changes. Looking into the bundle that the simulator runs, the size of the .cdm file didn't change as well.</p>
  
  <p>Another strange problem is that sometimes the the mapping model can't even be found, resulting in a migration error. In this case, the .cdm file in the bundle exists but is very small - less than 0.5k (my working file is ~14k).</p>
  
  <p>I've tried running "Clean", emptying Xcode's DerivedData directory and resetting the simulator. They solve the problem only for new mapping models, but editing them will again not work. This makes it virtually impossible to work with mapping models. Very frustrating :-(</p>