Add distgen interface

[jank324]

It came up that adding a Cheetah-native interface directly into distgen would make it very easy to generate all kinds of beam distributions.

This issue is to discuss the design of what that interface should look like.

Until this is implemented, it should be possible to do distgen -> OpenPMD -> Astra -> Cheetah.

[jank324]

So I would suggest to hit to birds with one stone here and add both an OpenPMD interface and distgen to ParticleBeam like in the following pseudocode.

@classmethod
def from_openpmd(cls, some_arg) -> ParticleBeam:
    # Some code to convert from OpenPMD to Cheetah
    ...

@classmethod
def from_distgen(cls, distgen_input: str | Path | dict) -> ParticleBeam:
    gen = Generator(generator_input, verbose=0)
    openpmd = gen.run()
    return cls.from_openpmd(openpmd)

I'm not familiar with distgen and this is only based on skimming over the examples. So maybe @cr-xu and @jp-ga do you have an opinion on this?

[jp-ga]

t to hit to birds with one stone here and add both an OpenPMD interface and distgen to

Sounds good to me. I did something similar with Bmad-X. See https://github.com/bmad-sim/Bmad-X/blob/main/bmadx/distgen_utils.py and https://github.com/bmad-sim/Bmad-X/blob/main/bmadx/pmd_utils.py