Background estimation with a custom callable statistic

[hpparvi]

This PR continues the work done in #255 and #253, enabling the user to provide a custom callable for background calculation. The primary changes are:

• The statistic argument in the Background initializer now accepts "average", "median", or a custom callable. The callable should accept a numpy.ma.MaskedArray as input and must also support an axis argument.

• The bkg_statistic keyword in Background.bkg_spectrum() is now deprecated and triggers a warning if used. Instead, the statistic argument in the Background initializer should be used.

I'd prefer to include these changes in a bugfix release (v1.5.2), which is why I've opted to deprecate rather than remove the bkg_statistic argument. However, the current implementation ignores the bkg_statistic argument, meaning that although the API itself hasn't technically changed, the code's behaviour has.

Let me know your thoughts on this approach or if you have suggestions for handling this differently.

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 2 lines in your changes missing coverage. Please review.

Project coverage is 87.61%. Comparing base (6b1887b) to head (20ff843).

Files with missing lines	Patch %	Lines
specreduce/background.py	85.71%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #263      +/-   ##
==========================================
- Coverage   87.74%   87.61%   -0.13%     
==========================================
  Files          15       15              
  Lines        1289     1284       -5     
==========================================
- Hits         1131     1125       -6     
- Misses        158      159       +1     

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