avoid call to redis when testing if object is a MeasurementGroup instance

When starting a scan, X calls to redis are to get active measurement group name ; this comes from the get_all method that tests each object from setup_globals to build the list of MeasurementGroup objects ; X being equal to the number of exported objects in setup_globals. So, fixing __class__ to return MeasurementGroup avoids all the calls to redis when dealing with the default measurement group object.