Generator-level info in `event` output tree

[mvicenzi]

The event ouput TTree sketched in #16 is effectively empty.

The plan in to fill it with generator-level information: this should include generator-level info: the type of generator used (genie, foresee\hepmc) and the physics input to that generator (e.g., decay or neutrino kinematics).
We can also add vertex information, as well as a vector of primaries.
This requires introducing a general way to store generator metadata that can work across the different generators we support.

My idea would be:

• create a new GeneratorMetadata struct or class, and use it as a private member of GeneratorBase;
• it should probably be a vector in order to support more vertexes in the same event (?);
• it should be flexible enough to hold info from very different generators: info on the interaction/decaying particle, identification of the process (neutrino interaction or BSM?), etc;
• find a way to fill the metadata.. somewhat easy for genie files, might be harder with hepMC files depending on how much is being stored in the event header
• save this metadata to a custom UserEventInformation so that it can be accessed at the end of the event by the AnalysisManager...

Comments/suggestions are welcomed!