GTFS importer

[munterfi]

Write a function int import_gtfs(Network *network, char *folder_path) to import GTFS schedules.

Documentations:

• GTFS Static Overview
• Example dataset for Timetable 2021 in Switzerland
  The timetable contains the basic topological and temporal elements, which allow a timetable display and information. The timetable is provided using the General Transit Feed Specification Format (GTFS).