Boxcar extraction with non-finite pixels

[hpparvi]

This PR adds a new exclude mask treatment option to BoxcarExtract, allowing the extraction to exclude any non-finite pixels. With the exclude option, the extraction is now carried out as a weighted sum, calculated as the average of the finite in-window pixels multiplied by the number of in-window pixels. The behaviour stays identical to the previous with the other mask treatment options or if no non-finite values are present.

[codecov]

Codecov Report

Attention: Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.

Project coverage is 85.44%. Comparing base (e0ac82a) to head (9713654).

Files with missing lines	Patch %	Lines
specreduce/extract.py	94.11%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #245      +/-   ##
==========================================
+ Coverage   85.38%   85.44%   +0.06%     
==========================================
  Files          13       13              
  Lines        1177     1182       +5     
==========================================
+ Hits         1005     1010       +5     
  Misses        172      172              

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