Use self.observatories to determine number of inputs

Previously use the number of traces in the input Stream. This could
be fewer than the number of desired observatories if a given input's
data was completely missing.

geomagio/algorithm/AverageAlgorithm.py

@@ -122,7 +122,7 @@ class AverageAlgorithm(Algorithm):
 
         self.outlocation = self.outlocation or timeseries[0].stats.location
 
-        self.min_count = self.min_count or len(timeseries)
+        self.min_count = self.min_count or len(self.observatories)
         self.min_count_start = self.min_count_start or timeseries[0].stats.starttime
         self.min_count_end = self.min_count_end or timeseries[0].stats.endtime