refactor gtfs module

[r-leyshon]

Description of the Feature to be Added

Slated for January, refactor of GTFS module:

• avoid circular import on type hinting
• renaming of modules & classes to clarify behaviours
• Implementing directory structure hierarchy for dependencies
• Abstracting away internals from the api classes
• Refactoring class naming so that GtfsInstance -> SingleGtfs and MultiGtfsInstance -> MultiGtfs as mentioned in PR 207

(OPTIONAL) Suggested Implementations

Additional context

Please see discussion board for more detail.

[r-leyshon]

Noting that single or multiple gtfs instance summarise_trips / routes behaviour currently diverge . Single Gtfs will return day summaries while multiple gtfs will return date summaries. Once we have abandoned the single gtfs instance approach, this problem becomes irrelevant.