lcio-ex-read-event command

lcio-ex-read-event is the hep/x/lcio example equivalent to:

https://github.com/iLCSoft/LCIO/blob/master/examples/cpp/rootDict/readEventTree.C

example:

$> lcio-ex-read-event ./DST01-06_ppr004_bbcsdu.slcio
lcio-ex-read-event: read 50 events from file "./DST01-06_ppr004_bbcsdu.slcio"
$> open out.png