Return the requested model.

        :param modelIdentifier: <str> model identifier
        :return: <object> model instance

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)