Support GenEventData

[HDembinski]

• Adds GenEventData class
• Adds read-only Numpy API for GenEvent

The GenEventData class is an alternative representation of the GenEvent. It is a different data structure, so converting between the two objects is not free of cost, but the cost is moderate.

The GenEventData class was introduced by the HepMC3 devs, because it has a simple layout that allows one to store the instances in ROOT trees. From the perspective of a Python user is also interesting, because it allows one to access and manipulate the particles and vertices in an event in form of numpy arrays, which are views into the memory of the GenEventData object. This is cheap and processing numpy arrays is fast.

In case of read-only access, one can do better than converting GenEvents to GenEventData objects. An alternative Numpy API was added to the GenEvent, which provides faster read-only access to particle and vertex data members.

If you need to process events in Python, you now have several options, see docs/examples/processing.ipynb

[coveralls]

coverage: 99.125%. remained the same
when pulling f038305 on geneventdata
into 6866c8b on main.