Background.bkg_spectrum and sub_spectrum

[kecnry]

This PR implements methods to expose the background (and background-subtracted) spectrum from a Background object.

bg = Background.two_sided(image, trace, separation, width)
bg.bkg_spectrum()
bg.sub_spectrum()

[codecov]

Codecov Report

Merging #143 (cade209) into main (78e68b7) will increase coverage by 0.54%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #143      +/-   ##
==========================================
+ Coverage   70.57%   71.12%   +0.54%     
==========================================
  Files           9        9              
  Lines         605      613       +8     
==========================================
+ Hits          427      436       +9     
+ Misses        178      177       -1     

Impacted Files	Coverage Δ
specreduce/background.py	87.67% <100.00%> (+3.29%)	⬆️
specreduce/extract.py	89.23% <100.00%> (-0.09%)	⬇️

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[ojustino]

These are logical additions that line up well with bkg_image. The core looks good to me, so the following comments are on peripheral issues.

[ojustino]

Since the only outstanding comment is on a private function, it shouldn't prevent a merge if others are ready for one.

(Note to self that this will cause some merge conflicts with #144)