Change default unit argument from "m" to "km"

[ethan-moss]

Description of the Feature to be Added

The GtfsInstance class has a default argument of "m" for the GTFS units. However, this is not very common - "km" is more widely adopted and would be a better choice as the default argument to help protect against potential unforeseen errors.

(OPTIONAL) Suggested Implementations

Change the default unit argument from "m" to "km" throughout the class.

Additional context

Examples of Km units being used can be provided if needed.

[ethan-moss]

Closing since performance gains would be too minor and unable to trust the stability of the end point.

[r-leyshon]

Reopening as realised this was not completed with #143 (my mistake, that resolved the default units value in the OSM module, not GTFS).