Add an header with the event size in the AMC13 readout data format

Summary

Currently, the bistream produced by the AMC13Readout application does not include header information about the size of an event, even though this size is know when reading out the event from the AMC13. Therefore, the unpacker has no possibility to know about the event size before parsing it.

Having a way to know the event size before parsing it should simply event unpacking and allow for block read optimization when working with variable size events (zero-suppression). The Python unpacker also expects the event size to be contained into a per-event header.

What is the expected correct behavior?

An header with the event size should be appended to each event.