Create a `Photon` class

[namurphy]

It would be really helpful to have a Photon class that would include the energy, frequency, wavelength, and momentum of a photon. This would behave similarly to a Particle object from PlasmaPy. To do the conversion between those physical quantities, it might be helpful to look into Astropy equivalencies. There's a bit more information in PlasmaPy/PlasmaPy#1175. Thank you!