Re-do of plot_quick

[eteq]

This PR is a re-do of #650 for the current state of affairs where ndcube's plot() method exists on Spectrum1D. After some testing, a group of us realized the ndcube plot doesn't really represent well what plotting should be doing for a Spectrum1D. So this PR moves plot_quick to override the .plot() of ndcube. A few others at the workshop will fill in a follow-on comment later with the experimentation that demonstrated to this group why this is a good idea.

[codecov]

Codecov Report

Attention: Patch coverage is 84.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 70.81%. Comparing base (ba0ab68) to head (4ef4dcb).
Report is 43 commits behind head on main.

Files with missing lines	Patch %	Lines
specutils/spectra/spectrum1d.py	84.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1106      +/-   ##
==========================================
+ Coverage   70.74%   70.81%   +0.07%     
==========================================
  Files          64       64              
  Lines        4484     4509      +25     
==========================================
+ Hits         3172     3193      +21     
- Misses       1312     1316       +4     

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