`atoms.todict()` returns non-JSON serializable representation

According to the docstring of Atoms.todict (https://gitlab.com/ase/ase/blob/35c0d79aa89b12636c0259b7b9f6874a3652074c/ase/atoms.py#L870-882), the following should be possible:

import json
from ase import Atoms

atoms = Atoms("N2", [(0, 0, 0), (0, 0, 1)])

dct = atoms.todict()
rep = json.dumps(dct)

However, this returns

TypeError: Object of type ndarray is not JSON serializable

I'll propose a fix.