Generic Image

Should we have a model for image data that is not holograms? I (tgd) at least sometimes end up using hp.Hologram as a generic image container and sometimes feel a little bad about it. Also, truthfully, scatterpy.theory.*.calc_intensity should not be returning holograms. Mostly a semantic isssue, but names should describe what they are.

I think a solution to this may be to have a parent class that is a thinner wrapper around np.ndimage that implements our image model and metadata on top of the basic ndarray. Reconstruction and hologram can then inherit from that parent.