Draw bounding box based on footprint if no pointcloud is available

[sverhoeven]

The sites.json now also contains the footprint_alitude min/max. The altitude was was derived from the points in the drivemap which are inside the site footprint.
The example json was updated in c2dc729

Now only bounding box are drawn for sites which have pointclouds. Almost all sites have footprints.
As a user I would like to have more boundingboxes.