add particle species in ParticleBeam

[jp-ga]

I have noticed that ParticleBeam doesn't have a particle mass property. It ends up being necessary to assume particles are electrons and then import the electron mass for different elements/tracking routines. It would be useful to add the particle mass or species to ParticleBeam and use electrons as default.

[jp-ga]

Any thoughts on this @jank324 @cr-xu ?

[jank324]

Any thoughts on this @jank324 @cr-xu ?

This was discussed somewhere before, I don't remember where. In the end we didn't do it (yet) because we didn't need this yet.

I don't see any reason why we shouldn't add it.

[cr-xu]

Yes, we should add this. It's just been on the to-do list for a while.