Come up with a more clear syntax of particle slicing

[RudolfWeeber]

It should probably be made clear by the syntax of
system.part[]
that ranges refer to particle ids, not indicies in a (probably sorted ascending) list of existing particle ids.
E.g.,
system.part[min_id=0, max_id=20]
system.part[min_id=5]

[jngrad]

The []-operator was removed by #4402. Closing.