Upgrade2-Reconstruction developments guidelines

As initiated in !!4288 (merged) , we should define some guidelines on "where to put classes" we use for reconstruction within Rec if we migrate some event model to Rec for prototypes.

Current proposal from @tevans is :

Event/FutureEvent/

containing the changes we do on "core-event" objects we aim to persist in some form or to be shared between tracking-calo-pid tasks.

and

Pr/PrEvent

to contain the classes used within the pattern recognition implementations.

Feel free to comment on this issue to help forming guidelines. Other suggestions are about the usage of namespaces for U2 reconstructions.