Allow phonon data to be loaded

[ElliottKasoar]

Currently, I don't think we provide a way to reuse phonon output files e.g. if we want to calculate more properties through the CLI, in the same way that we can run each type of calculation separately in the Python interface.

I think we should be able to use phonopy.load to do this.

[alinelena]

i think if you load phonopy.yaml shall be ok.

[ElliottKasoar]

i think if you load phonopy.yaml shall be ok.

Sorry, as in you think we can already do this, or you're saying that's how we'd do it?

[alinelena]

yap i think we can do it.. not via cli. let us test it

[ElliottKasoar]

Ok yep, I'm mostly thinking about the CLI, but for the Python perspective, pretty sure we'd need to set self.results["phonon"] to the loaded object, if we wanted to use it as if we'd calculating it ourselves, so a helper wouldn't hurt.