Improve header printout
The header shows a rather unhelpful string when dumped into the REPL:
>>> f = km3io.OfflineReader("../km3net-testdata/km3net_testdata/data/offline/empty_events.root")
>>> f.events.header
<km3io.offline.Header at 0x16896b8e0>
It would be good if it would show the fields and maybe even values.