Simplifying and clarifying text output from change avg speeds script.
slight print statement improvement.
* Improving the way tuple values are referenced by using const variables instead
of row numbers directly.
* Committing needed function
* get_nearest_next_valid_freq_and_time(service_headways, time_of_day)
  - used in timetable production where a service headways file might not have
    a valid value in all time periods.