Small bug fixes and upgrades related to Interstage (final update)

[carlandreaslindstrom]

This PR is a final "cleanup" for the interstage work.

• It contains a bug fix in the beam_physics utility, now dividing instead of multiplying by the factorial in higher-order dispersion calculations (the second-order dispersion is used a lot and was off by a factor 4).
• Visual improvements to the interstage optics and layout plots, including adding drivers.
• Some additional control parameters of density plots in Beam.
• Added parallel loading of data in Runnable (used in interstage paper figures).
• Renamed the isr_on_ref_part parameter in the ImpactX interstage (to be more self-consistent).
• Removed the ISR kick mitigation offsets (not handled by adding ISR directly to the reference particle).
• Fixed bug in the ImpactX interstage where the second lens was overwritten by the first.

[carlandreaslindstrom]

I will just merge these now that the tests are passed (they didn't before, so some bugs were fixed).