Use an OSM route as a route

[pyrog]

Related to #312

Currently, one could download an OSM relation route in gpx format from way marked trails , and upload it in brouter.

Example :

• hiking route 1213545 in WMT
• gpx file

brouter web and/or brouter API could use OSM relation directly.

[nrenner]

The routing data only knows about network type on a way, not about individual routes, so we would need to get the route geometry from an external service like Overpass API. Waymarked trails explicitly doesn't allow other services to use their data.

The other question is how to select an OSM route. Ideally by clicking on the map, which would again need an external vector tile service (all commercial) or querying the Overpass API. A simple alternative would be entering the relation id or drag&drop of an URL with the relation id.

Rather sooner than later we will probably need to use commercial services or setup our own for such things, which both requires money (and additional maintenance effort).