Event list

This is an implementation of the TEventList speedup that we discussed a few weeks ago.
I tested it on one MC sample, and it works correctly.
There is one example showing its usage.
In short:
- add a EventlistHandler member to your analysis class
- initialize its parameters when you run (mainly the name of the root file where the list is stored)
- within `Process` mark the events you want to save in the list
- on subsequent runs, the list is picked up (rm the cache if you want to reset it, for example when you change your selection)