2 - filter POI by portal

[IdrissaD]

Treks can be filtered by portals, but not POI (cf. GeotrekCE/Geotrek-admin#3043). One way to get roughly the same effect would be to spatially filter POI, assuming portals in the source DB have a spatial meaning (organizations that don't cover the same area).

Add possibility to enter a BBOX to filter POI directly in the API:

• pros: less data fetched, performances improved;
• cons: BBOX is not accurate

Add spatial filtering between API fetching and data processing:

• pros: quite accurate spatial query, performances improved in data processing
• cons: all POI are still downloaded

Another option would be to only download POI that are linked to a trek we fetch, through API poi/?trek=XXX route:

• pros: filtering can't be more accurate
• cons: from a performance point of view, this leads to as many API requests as there are treks...which is disastrous