Helper on circular buffer

[bertrandlalo]

The current implementation may miss some initial samples and send too many at the end if the event stream and data stream are not perfectly in sync.
A solution would be to maintain a one-second or so circular buffer even if the gate is closed.
Since this is already done in node epochs, it could worth a helpers.

timeflux/timeflux/nodes/gate.py

Line 58 in b0a0981

self.o = self.i