Replace sedkit.Spectrum with specutils.Spectrum

[kelle]

Lots of functionality in spectrum.py should be replaced with specutils.
https://specutils.readthedocs.io/en/stable/

[hover2pi]

This is doable but I suspect will be a LOT of work. Do we get more accurate results with specutils or is this more about UX? I'm not opposed, just wary.

[kelle]

First, this is definitely not high priority.

It's about sustainability. There are many tests for the functionality of Spectrum which are handled by specutils (trim, add, normalize, etc).

For folks potentially wanting to contribute, they are more likely to be familiar with the specutils.Spectrum class.