Add data class + unpacking macro for the SIS18 timing events

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The events are stored in an ASCII file during data taking, with an ad-hoc format, hence the decoder in the data class. The idea is that later we could have a "direct to binary" storage re-using the same class.

Given that the rate of messages that we use for now is in the 1-10 Hz area, I hope the current unoptimized data format is still ok. If we decide at some point to decrease the filtering and store more event types or if it appears that the frequency increases for the SIS100 case, the data class will need a rewriting.

Draft status: To be rebased (should be trivial) and merged only after !815 (merged) is merged