Added an implementation of attributes to GenEvent
Added an implementation of attributes to GenEvent to persistent classes. The changes constitute a new version 6 of the storage. Otherwise the content is equivalent to the version 5. So far the version 5 should be the default.
Tag @ewelina @jchapman @cgutscho @jkretz @hmildner @gpanizzo
@jchapman , will you have a look?
Some more notes:
- The v6 is in development phase.
- The v6 so far has more members than v5 -- to store the attributes.
- In the HepMC3 version there is a potential to save some space for the event size. One can drop the vertex weights, color flows and polarization members and save them as attributes only in case those exist.