Refactor instrument specific reading code to live on the associated spectrogram class

[samaloney]

Describe the feature

Spectrogram was heavily inspired/based off sunpy map where the sources are nearly always fits files or similar so it made sense to have centralised code to read these. The radio data comes in an array of different files/formats custom binary, fits, txt, and cdf for example and even in the same format the data may be stored differently. It would be better to move the instrument specific code to the instrument specific spectrogram classes, then the spectrogram_factory only does enough to figure out which instrument the data are from and passes the data off.

Proposed solution

No response