Create and process external MDC events to test RAVEN pipeline; fixes #401 and #403

These changes create and upload external MDC events in order to regularly test the fully RAVEN pipeline. This is designed to create an external MDC event for every 2 MDC superevents, resetting every day. These joint coincidences should be able to be found by ligo-raven>=2.0 by including search='MDC' in ligo.raven.search.search().

The MR includes changes to:

• create MDC external events via first2years_external.py
• use se_search field for superevent searches with new release of ligo-raven
• update superevent with coinc FAR values when better coincidence is available

gwcelery/tasks/first2years_external.py

@@ -16,7 +16,7 @@ def create_grb_event(gpstime):
     
     kwargs = {'mode': 'w+b'}
     with tempfile.NamedTemporaryFile(**kwargs) as xmlfile:
-        root = etree.parse(r”../tests/data/fermi_grb_gcn.xml”)
+        root = etree.parse(r"../tests/data/fermi_grb_gcn.xml")
         
         # Change times to chose time
         root.find("./Who/Date").text = str(new_date).encode()
@@ -44,10 +44,10 @@ def upload_external_event(alert)
     """
 
     if np.random.random() < .1:
-        gpstime = float(alert['object’][’t_0’])
+        gpstime = float(alert['object']['t_0'])
         new_time = offset_time(gpstime)
 
         ext_event = create_grb_event(new_time)
 
         # Upload as from GCN
-        external_triggers.handle_grb_gcn(ext_event)
\ No newline at end of file
+        external_triggers.handle_grb_gcn(ext_event)