Possibility to use TFileService file in TreeProducers and one simple fix in JetAnalyzer

with this if a tfileservie named "outputfile" is present in the services, the NumpyTreeProducer (and derived) would use such file rather than creating its own.
It also changes the standard behaviour to create the file at beginLoop rather than at init time (as the services are known only at begin loop)

@gpetruc