O4a online dtdphi plotter

This patch allows the following dtdphi plot to be uploaded to gracedb page.
Example G-event pages:
MDC 3-ifo event (two dtdphi plots)
MDC 2-ifo event (one dtdphi plot)
MDC 1-ifo event (no dtdphi plot)

TODO:

• test with single triggers
• test with online small analysis
• try adding LV plot and see if that is informative any way

gstlal-inspiral/bin/gstlal_ll_inspiral_event_plotter

@@ -517,6 +517,9 @@ class EventPlotter(events.EventProcessor):
 		ifos = sorted(event_kwargs["snrs"].keys())
 		ifo1 = ifos.pop(0)
 		snrs = event_kwargs["snrs"]
+		# remove segments for ifos not having horizon distance after
+		# compression, which would cause an error inside
+		# local_mean_horizon_distance()
 		segs_nonzero = {ifo:seg for ifo, seg in event_kwargs["segments"].items() if len(rankingstat.numerator.horizon_history[ifo]) > 0}
 		horizons = rankingstat.numerator.local_mean_horizon_distance(segs_nonzero, template_id = event_kwargs["template_id"])
 		figs = {}