openPMD-beamphysics ParticleGroup compatibility

[ChristopherMayes]

The structures here for describing a particle distribution should be compatible with the openPMD-beamphysics standard:
https://github.com/openPMD/openPMD-standard/blob/upcoming-2.0.0/EXT_BeamPhysics.md

Notably, a weight array should always be included.

See https://github.com/ChristopherMayes/openPMD-beamphysics for an implementation of a ParticleGroup structure in Python.

[mattsignorelli]

The only thing we seem to be missing is weight and t

[DavidSagan]

Stuff like weight actually does not have to be present for OpenPMD compatibility. But that said, we do want to have this field.