Add IGNF Scan25 baselayer (requires key)

[sylvainavelo]

Adds the IGN France - Carte topographique (Scan25) base layer.

This layer requires a valid key for "ignf" (see key.template.js in the root directory)

Key can be generated by:

• creating an account on https://geoservices.ign.fr/ (website is entirely in french)
• once logged in, visit "Mon Espace" > "Mes clés de service web" (https://geoservices.ign.fr/mes-cles)
• choose "Créer une clé"
• choose the first license "Usage gratuit" (free use)
• copy and use your new key
• optionally (recommended in production), add a referer safety on your key, so that only your website can send requests using it.

---

Related issue : #463

[nrenner]

Thanks!