Feature: add new geoportail layer for skitouring routes

[brunobesson]

Similar to the one available for swisstopo.

[eddy-geek]

xref

• Les itinéraires de Skitourenguru sur Géoportail - Forum - Camptocamp.org
• YETI commit

[eddy-geek]

by the way, I don't see the SwissTopo routes available either in Yeti or topo... and skitourenguru covers other countries but we use IGN layer so we miss them. So current state of winter route layers:

layer	in topo	in YETI
skitourenguru FR (ign)	✕	✓
sitourenguru IT, CH, AT	✕	✕
swisstopo	✕	✕

ideally, do we want to have all of this?

[fbunoz]

I think the 3 layers are useful.