Make datastore writer ignore old events

Some data provided to the writer will have been uploaded to Nikhef,
where the wsgi app was not as strict with marking events as suspect.

writer/store_events.py

@@ -111,13 +111,17 @@ def data_is_blob(uploadcode, blob_types):
 def store_event_list(data_dir, station_id, cluster, event_list):
     """Store a list of events"""
 
+    minimum_year = 2020
     prev_date = None
     datafile = None
     for event in event_list:
         try:
             timestamp = event['header']['datetime']
             if timestamp:
                 date = timestamp.date()
+                if date.year < minimum_year:
+                    logger.error(f'Old event ({date}), discarding event (station: {station_id})')
+                    continue
                 if date != prev_date:
                     if datafile:
                         datafile.close()