Gtfs-kit

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- Bugfix: updated function ``map_trips()`` to heed the ``include_arrows`` parameter.

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- Modularized some by added the functions ``build_aggregate_routes_dict()`` and ``build_aggregate_stops_dict()``.

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- Bugfix: updated ``aggregate_stops()`` to handle parent stations.
- Added optional direction arrows to ``map_trips()``.

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- Improved the fallback algorithm in ``append_dist_to_stop_times()``. Changed the function signature, so this is a major change, hence the major version bump.
- Removed optional direction arrows in ``map_trips()``, because the PolyLineTextPath Folium plugin needed for that seems to be broken.

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- Bugfix: imported ``aggregate_stops()`` as a Feed method.

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- Added ``aggregate_stops()`` function.
- Added optional direction arrows in ``map_trips()``.