bkg_spectrum to take func instead of string

[pllim]

This is a direct follow-up of unreleased feature added in #253 so behavior change can be done without deprecation or change log.

I am just wondering what it would have looked like.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.84%. Comparing base (c31a9af) to head (4894825).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #255      +/-   ##
==========================================
- Coverage   85.93%   85.84%   -0.09%     
==========================================
  Files          13       13              
  Lines        1166     1159       -7     
==========================================
- Hits         1002      995       -7     
  Misses        164      164              

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[kecnry]

If we do this, I think we'd want to mirror that in the class-level statistic parameter, but that has additional logic and restriction to just median/average, which might be difficult to make fully general 🤷‍♂️

[pllim]

Where is the statistic? I was looking at Spectrum1D.collapse upstream which takes function.

[kecnry]

In Background (see https://github.com/astropy/specreduce/blob/main/specreduce/background.py#L41, for example)

[pllim]

Hmm... Should #253 have used this statistic instead of introducing a new keyword?