Include event BPulseStPrev

That event will be used on a cycle before cycles with BPulseSt

init/sceconfig-ess.yml

@@ -69,6 +69,8 @@ Events: #[event codes]
   BiAcqSt: 18
   ## the following events are additional ones to be sent AHEAD of main evens but with TT controllable delay
   BiAcqStAhead: 19
+  ## The following event is produces on cycles previous to the ones with BPulseSt
+  BPulseStPrev: 20
   ## the following event is used by SCL RF systems to start the piezo motors
   RfPiezoSt: 22
   ## these, since they are derived from the original events, we apply code 90 + originalcode