Add new options to genesis4_par_to_data: wrap, z0, equal_weights, cut…

[ChristopherMayes]

This adds new options to genesis4_par_to_data.

• wrap: enables modulo-wrapping of theta
• z0: starting z-position offset
• slices: extract specific slice indices
• equal_weights: resample particles to uniform charge
• cutoff: skip sub-electron macroparticles
• rng: user-supplied or seeded random number generator

ParticleGroup now uses that to load Genesis4 particles directly with these options: