specify interval over which min_count is applied to AverageAlgorithm

Add ability to specify an interval (start and end) over which the recently added min_count option to the AverageAlgorithm is applied.

geomagio/algorithm/AverageAlgorithm.py

@@ -164,9 +164,7 @@ class AverageAlgorithm(Algorithm):
 
         # apply min_count_start and min_count_end
         # NOTE: the logic here is not very intuitive, but it works as intended
-        utc_datetimes = numpy.array(
-            [timeseries[0].stats.starttime + t for t in timeseries[0].times()]
-        )
+        utc_datetimes = timeseries[0].times(type="utcdatetime")
         average_data[
             (count_data < timeseries.count()) & (utc_datetimes > self.min_count_end)
         ] = numpy.nan