Add charge in the Beam classes

[cr-xu]

• Add total_charge to beam classes
  • ParameterBeam now has a total_charge attribute
  • ParticleBeam has now q_array which contains the charge of each particle in the particles. The total_charge is derived as sum(q_array)
  • Now the charge array will be also converted when importing from ASTRA or Ocelot beam.
• Update tests

The charge is tracked through the elements along with the beam. For now, the only element that affects the beam charge is the Aperture which removes the particle charges with large position offsets.

[jank324]

I would propose giving a better name to q_array ... something more like particle_charges.

Also, can you add the feature to the changelog?

Otherwise, this looks good to me.

[jank324]

Another thing that I'm just realising: Are the new charge properties properly tracked through all the elements?