Particles.copy() does not properly copy particle.lost_particles

[roman-martin]

import pysixtrack
import numpy as np

p1 = pysixtrack.Particles(state=np.array([0,0,1,1]))
p2 = p1.copy()

assert len(p1.lost_particles) == 0
assert len(p2.lost_particles) == 0

p1.remove_lost_particles()

assert len(p1.lost_particles) == 1
assert len(p2.lost_particles) == 0

Although we don't do anything with p2 after copying it from p1, cleaning up p1 changes p2.lost_particles.

This is because in Particle.copy() we only do a .copy (or slice copy) if the __dict__ item is of type np.array or dict. Unfortunately particle.lost_particles is of type list. Now we could include lists (an other mutables) to the check. However, the index that is also used, means something VERY different for x, px, etc than it does for lost_particles.