[Feature Request]: Make OpenBabel optional for molecule metadata

[Andrew-S-Rosen]

Problem

In emmet-core<=0.84.2, it was possible to make MoleculeMetadata without relying on OpenBabel as a requirement. However, in emmet-core==0.84.3rc4, OpenBabel is now implicitly a requirement because it is used in making the species hash.

OpenBabel is a very annoying dependency to require because it cannot be pip installed. It would be very nice to maintain the old behavior and support the creation of MoleculeMetadata without OpenBabel as a dependency. I propose that we generate a different hash if OpenBabel is not around (nor not make one at all).

from pymatgen.core import Molecule
from emmet.core.structure import MoleculeMetadata

mol = Molecule(['H','H'],[[0,0,0],[1,1,1]])
MoleculeMetadata.from_molecule(mol)