Allowing to have collections of event files as event data

[sponce]

Currently the event data are read from a single event file which is fully loaded at start time. This means that all events are present in memory, potentially taking quite some space.

One could think of being able to have several event files as input, and only load one in memory at a time so that we can drop previous events when a new file is loaded.

It could even be expanded to collections of files (several collections, maybe even with a hierarchy in a tree) so that we could expose to the users several sets of files, e.g. "Best events", "B physics", "Live events", etc... and the user can choose what (s)he likes to see at a given moment.