Export EPSG:4326, not EPSG:3857

[MoritzStefaner]

All GeoJSON files carry "crs": {... "EPSG::3857"} and Web-Mercator metre coordinates. RFC 7946
mandates WGS84 lon/lat and forbids the crs member, so no standard web mapping library reads them
as-is. We reproject on import.

Ask: write RFC 7946 GeoJSON. RFC7946=YES drops the crs member and sets
7-decimal precision.

gdf.to_crs(4326).to_file(path, driver="GeoJSON", RFC7946="YES")

Note: Coordinates currently arrive rounded to whole numbers. The same rounding applied to degrees
would be a ~100 km error, so please don't switch CRS without raising precision.

SuperBlockify already writes UTM per city (EPSG:32631 for Brussels), which is metrically correct
and just needs the same conversion for the web.

Fix status:

• growbikenet (@mszell)
• fixbikenet (@mszell)
• linkbikenet (@Manuel-Knepper, after conferring with @mszell)
• superblockify (potentially @cbueth , otherwise @marianamirandapessoa with a conversion script)

[mszell]

Thx, great to know!
I'll start fixing this in gbn. After I'm done, I'll let you know @Manuel-Knepper for the copy-pasta. (In the future we will have one single lib for IO etc, but we are not there yet..)