Include extras in get_hash function of Molecule

[awvwgk]

Is your feature request related to a problem? Please describe.

When staging new features in extras like a lattice (see MolSSI/QCSchema#66), the extras field is not included in the hash, making a molecule periodic box equivalent with a molecular system.

Describe the solution you'd like

Include the extras dict in the molecule hash

Describe alternatives you've considered

Provide a way to add additional fields to the hash for a Molecule.

Additional context