p0c setter bug

[kparasch]

part1 = pysixtrack.Particles(p0c=2e10)
part1.tau=1.e-4
part1.ptau=0.

part2 = pysixtrack.Particles()
part2.p0c=2e10
part2.tau=1.e-4
part2.ptau=0.

print(part1.tau)
print(part2.tau)

part1.tau -> 0.0001

part2.tau -> 7.300582693553491e-05

[roman-martin]

Reasons this happens:

p0c.setter uses _update_ref() and _update_particles_from_absolute() which do not update rvv (and some other stuff like zeta),
then ptau uses the delta.setter, which does not update zeta.