Monitor: define groups by Overpass queries

[StC-OSM]

Groups are extremely useful: they make it possible to find one's way in the huge number of existing routes. Still:

• they are fairly costly to create and manage;
• they sometimes rely on relations that are more or less lists, e.g. the national segments of E* and EV* routes

By adding support for Overpass-defined groups, we could probably avoid the two above issues. Say we remove the national segments from E* and EV* and replace them with careful management of the operator tag in sub-relations; we could create a group of "French sections of EV*" by requesting all sub-relations of routes named "EuroVelo .*" that have the appropriate operator tag (or any other relevant tag).