Associate POIs with trails

[cbroms]

For now, includes:

• Lakes (within 50m)
• Rivers (within 50m)
• Waterfalls (within 100m)
• Peaks (within 100m)
• Glaciers (within 50m)

Each POI should have an associated ID. These should be saved on each trail (e.g. near_lakes: [jlva3414sd, 3nl134jsxa])