Merge branch 'empty-input-channels' into 'master'

add_empty_channel = True for derived inputs

See merge request !135

geomagio/DerivedTimeseriesFactory.py

@@ -99,7 +99,7 @@ class DerivedTimeseriesFactory(TimeseriesFactory):
                 channels=missing_inputs,
                 type=data_type,
                 interval=interval,
-                add_empty_channels=False,
+                add_empty_channels=True,
             )
         output_timeseries = Stream()
         for channel in channels: