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

@@ -38,13 +38,13 @@ def create_grb_event(gpstime):
     root.find(("./WhereWhen/ObsDataLocation/"
                "ObservationLocation/AstroCoords/Position2D/Value2/"
                "C1")).text = str(np.random.uniform(0, 360)).encode()
-    thetas = np.arange(-np.pi/2, np.pi/2, .01)
+    thetas = np.arange(-np.pi / 2, np.pi / 2, .01)
     root.find(("./WhereWhen/ObsDataLocation/"
                "ObservationLocation/AstroCoords/Position2D/Value2/"
                "C2")).text = \
         str(np.random.choice(np.rad2deg(thetas),
                              p=np.cos(thetas) / sum(np.cos(thetas)))
-        ).encode()
+           ).encode()
     root.find(("./WhereWhen/ObsDataLocation/"
                "ObservationLocation/AstroCoords/Position2D/"
                "Error2Radius")).text = str(np.random.uniform(1, 30)).encode()