Exception to throw when a data file for the Demographic
Forecasting system is incorrectly formatted for the
current model (thrown by use of FileTools.bad_format()).
Implementing classes are expected to override this method as a
way to update their characteristics objects if necessary
before each increment of the model.
Stores all of the results from the model,
and provides access to the results of each year for
which the model has been run, plus the final populations.