update interval no longer shrinks with recursion

Previously, the endtime-starttime interval being processed by the run_as_update method would shrink by 1 (second) with each recursion. This ultimately broke run_as_update when processing anything other than 1-second data, but it wasn't obvious because we rarely had to actually recurse. There is a little trickery now to ensure that user- provided starttime and endtime are inclusive of the full first (most recent) update interval, while subsequent, calculated, endtimes are set equal to the prior starttime minus specified output_interval (e.g., 'second', 'minute', etc.).