Fix qualified business income calculation

[martinholmer]

The 4.2.0 version of Tax-Calculator does not calculate qualified business income (QBI) correctly when the business income is from self-employment (that is, reported on Schedule C). The reason for the miscalculation is that the above-the-line deduction for the self-employment (SECA) tax on Schedule C income is not being subtracted from Schedule C gross income to get "Net QBI". Here are two IRS forms as generated by 2022 Turbo Tax for a tax unit in which the head has $200,000 in Schedule C income and qualified business income of $188,208 with the difference being the $11,792 above-the-line deduction for the self-employment (SECA) tax.

First the form that computes the self-employment tax:

.

Then the QBID (Form 8995) summary:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.42%. Comparing base (0b775e5) to head (5621d50).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2785   +/-   ##
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  Coverage   99.42%   99.42%           
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  Files          13       13           
  Lines        2613     2613           
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  Hits         2598     2598           
  Misses         15       15           

Flag	Coverage Δ
unittests	99.42% <ø> (ø)

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[jdebacker]

@martinholmer Thanks for finding and fixing this bug. Merging changes.