Read charges from lammps dump

If charges are present in the lammps dump file, neither trajectories nor snapshots can be visualized. The point is that charges are only scalar property, which is read by get_quantity(). Thus the charges were returned as [[q1], [q2], ..]. This is fixed as well as the test has been added.

The similar MR can be closed after merging this one.

closes !2387 (closed)

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