rework EventLoop output streams

The goal is to unify the output stream handling, both between the different drivers and between the histogram and the n-tuple output. There are more merge requests to come, the main goal of this merge request is to ensure the user can specify an output stream for histograms like he does for n-tuples (if he so chooses).

Part of the motivation is to address this JIRA by allowing to redirect the histogram output to a different storage element: https://its.cern.ch/jira/browse/ATLASG-1476