docs: def getModel(self, modelIdentifier):
          """
          Return the requested model.
  
          :param modelIdentifier: <str> model identifier
          :return: <object> model instance
          """
          if modelIdentifier in self._models:
              return self._models[modelIdentifier]
          else:
              message = "Application - getModel() - " \
                        "Model with identifier {} does not exist." \
                        .format(modelIdentifier)
              raise Exception(message)