Handle GPX decode

Hi!

Here is a first working (and unittested) version of GPX decoding.
To notice:
- routes are not handled, only waypoints and tracks; I don't know what to do with routes: they are a list of **ordered** waypoints, so I don't see what GeoJSON type to use
- only the first `trkseg` is used for a track, maybe it could be better to loop over every elements, but this will result in a `LineString` not exactly has the one processed. The real ideal solution is to map it with a `MultiLineString`, but I don't think that vectoformat is using `MultiLineString` at the moment

Thanks!