New particle access syntax

[christophlohrmann]

Fixes #4389

Description of changes:

• The []-operator on system.part was removed
• An individual particle can be accessed by id using system.part.by_id(id), where id is the numerical particle id. This returns a ParticleHandle
• A slice of particles can be accessed via an iterable of ids with system.part.by_ids(id_list), which returns a ParticleSlice
• system.part.all() returns a slice containing all particles
• The order in which particles will be provided when iterating over a slice will stay fixed, once the slice has been instanced. I.e, for pos, vel in zip(system.part.all().pos, system.part.all().v) will produce the expected result.
• Selection by criterion (either using key=value or a lambda) can be done with the existing system.part.select()

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[jngrad]

Thanks for the hard work!