Change filter to return events partially contained in start/stop

Previously `filter(start, stop)` returned events that were fully contained within start : stop.  Now it returns events that are partially contained within the interval.  I.e. events that event partially intersect start / stop are returned.  In this way something like the following gives a useful result:
    
    >>> events.dwells.filter('2012:002:00:00:00', '2012:002:00:00:01')