text: Title: Move Model documents to different files (MongoEngine) Example
  Body: I am trying to figure out how I can create the `MongoEngine.Document` classes in separate files and still use the instance variable here:
  
  https://github.com/biosustain/potion/blob/dc71f4954422f6edfde5bfa86f65dd622a35fdea/examples/mongoengine_simple.py#L12
  
  Is there a good way of doing this so I can create a connection to the database and pass that mongo engine object around when i define my class modules in separate files?
  