Summary

Closes #3668.

The Atoms.info dictionary can be populated with all kinds of goodies that are challenging to deal with from a (de)serialization perspective. Oftentimes, this is from users hacking things into Atoms.info. For instance, in #3668, I put a Structure object in the Atoms.info dictionary, which then broke things during serialization. In order to ensure MSONAtoms plays nice with the (de)serialization routines in monty, it is better to split off the Atoms.info metadata into its own entry in the .as_dict() dictionary that we can sanitize ourselves. That's what I've done in this PR.

The following toy example now works in a round-trip fashion:

from pymatgen.core import Structure
from monty.serialization import dumpfn, loadfn

structure = Structure(
    lattice=[[0, 2.13, 2.13], [2.13, 0, 2.13], [2.13, 2.13, 0]],
    species=["Mg", "O"],
    coords=[[0, 0, 0], [0.5, 0.5, 0.5]],
)
mson_atoms = structure.to_ase_atoms()
mson_atoms.info = {"test": "hi", "s": structure}
dumpfn(mson_atoms, "test.json")
loadfn("test.json")