Support returning atom-resolved energies

[awvwgk]

Most calculations internally use atom-resolved energies, which would make it rather straight-forward to expose those quantities to the user via an optional output or via an API function.

Care should be taken to not break the API and also not require the external integrations to account for atom-resolved energies. In case only a total energies is required, the usage of atom-resolved energies should remain an implementation detail.