Transition model- handle agent segments of length zero and certain control total conventions

Control totals tables that are heavily segmented may imply agent types that do not exist.  Instead of stopping the simulation, we can log a message notifying the user of the nonexistence, then continue.  Also, certain changes were made to the _filterize function in utils so as to handle existing urbansim conventions regarding control total table format.  These changes were motivated by use of SEMCOG's household control totals in full detail.
That is a good point.  This has not showed up, as we would bin the continuous variables into discrete categories before passing to the transition model.  (e.g. income_group instead of income).

I like the new way, and the ability to use continuous variables directly in the control total table.  I'll add another pre-processing step to the current transition model I'm working on.