Fix processes streamer

[jgalan]

see: https://root-forum.cern.ch/t/checkbytecount-and-streamer-error/52716/58

Fixes issue #348 for axionlib event data processing. However, there are still problems under certain conditions. When processing restG4 data, processes will not be present on the streamer. See new issue created to track this problem #358

• Adds a validation pipeline in TREX to check for the number of processes written to the streamer. Still there are some problems here, not all the processes streamer are properly written to disk. See for example the processes that should be present after 03_hits.rml, TRestDetectorSignalToHitsProcess and TRestDetectorHitsAnalysisProcess are not present!