Add default and set to zero masking option for specreduce operations.

[cshanahan1]

(incomplete draft, just look at tracing and background for now)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 37.20930% with 54 lines in your changes missing coverage. Please review.

Project coverage is 45.56%. Comparing base (6b8e995) to head (d4d528d).

Files with missing lines	Patch %	Lines
specreduce/core.py	18.91%	30 Missing :warning:
specreduce/tracing.py	58.33%	15 Missing :warning:
specreduce/background.py	22.22%	7 Missing :warning:
specreduce/extract.py	50.00%	2 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #216       +/-   ##
===========================================
- Coverage   83.37%   45.56%   -37.81%     
===========================================
  Files          13       13               
  Lines        1137     1196       +59     
===========================================
- Hits          948      545      -403     
- Misses        189      651      +462     

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[tepickering]

finally got a chance to at least look through the code changes. i like it so far! let me know when it's ready for a fuller review and i'll try playing with it in more depth.

[camipacifici]

I am pretty sure it is! Please go ahead. Thank you!

[tepickering]

there are a bunch of warnings in the RTD build. they look like malformed references, but need a deeper look to see what's the actual cause.

[tepickering]

i'm confused why python 3.8 and 3.9 tests are being run. a rebase might be needed, though i thought this was forked after those tests were removed.