Run Mode: Only Reference Particle

[ax3l]

For a couple of workflows, e.g.,:

• converting lattice files that use focusing strengths (relative to the magnetic rigidity)
• determining phases to RF cavities (phase scan)

a first run of the reference particle as single particle would be helpful to determine properties like reference beam energy over s.

Maybe we can make this convenient, e.g., similar to sim.evolve() a first, optional sim.reference_evolve() that just pushes one particle.

• ASTRA calls this "auto_phase". The option "phase_scan" then uses information like this to adjust arrival times in cavities or other optimizations (?).
• Elegant calls this "fiducialization" (which is their way to calculate a reference orbit from bunch properties).