Open bbox_filter_gtfs() bbox to be a polygon

[ethan-moss]

Description of the Feature to be Added

Open bbox_filter_gtfs() bbox to be a polygon instead of a list:

• GTFS kit docs suggest the restrict_to_area() method accepts a gdf of polygons. So we could open up this function to accept a polygon as an input (instead of a BBOX). This would then be the same input type as the RasterPop module.
• This would also the work better with the urban_centre module, which returns shapely polygons.

[r-leyshon]

Sounds like a useful feature @ethan-moss , though could we have a list or a polygon? Common pattern of mine is to pull a bbox csv of klokantech etc and paste it into a list.