[Feature Req.] power spectrum function in py21cmfast

[NGillet]

It would be comfortable to have a power spectrum function in py21cmfast for the lightcone and coeaval object

[steven-murray]

This could then also be used as a callback in either run_lightcone or run_coeval.

[steven-murray]

Will need separate functions for lightcone and coeval. Lightcone is a bit more involved.