Proposal to fix #13640

This adds a new enviornmental variable OONI_MAX_DISTANCE_CM which can
be used to specify the maximum number of hours that the control
measurement may differ from the actual measurement.

The new measurement status "inconclusive" is introduced which means that
no control measurements that fullfills the distance requirements was found.

Additionally find_closest now returns None if no control measrements could
be found.
Merge pull request #11 from olliwolli/ticket-13640

Ticket 13640