Batch particle properties update

[NiklasMueller2001]

Until now the update() method of the ParticleHandle class just sequentially set its parameters to new values, making one expensive update call for each new parameter. Now a batch update is performed.

[jngrad]

Overall looks good to me. I'm currently working on this PR to deduplicate some of the common code.

Is there a particular reason to prefer the current syntax

p.update({"bonds": new_bonds, "pos": [1, 2, 3]})

over the following syntax?

p.update(bonds=new_bonds, pos=[1, 2, 3])

The latter is closer to the syntax of system.part.add(), and wouldn't require additional_properties to store overridden values. The function would take **new_properties as argument, so it would be safe to override a value without affecting the parent scope. I had to look up PEP 584 to double-check the pipe operator uses the last occurrence of a key/value pair in the return value (nothing against it, the old union syntax with expansions is also cryptic in my opinion, but removing unions makes the code easier for newcomers).

[jngrad]

I see: the system.part.update() takes a dict as argument, so we mimic that in the particle class.

[jngrad]

Thanks!