Collection discovery BBOX filter not available

The bbox filtering is, at the moment, only available for data search within collections, and not for the collections discovery.

By way of example, the following request, that searches for collections inside a certain bbox:

https://hda.central.data.destination-earth.eu/stac/collections?bbox=72,71,74,72&datetime=2017-07-05T00:00:00Z

is, currently, returning collections which are not matching the bbox specified in the request.