Add taxable-income cap to QBID logic

[martinholmer]

This pull request fixes a bug in the qualified business income deduction (QBID) logic added in pull request #2345. That earlier pull request failed to model the taxable-income cap on the QBID amount. For information on that cap, see page 2 of the March 13, 2019, JCT briefing entitled "Overview of Deduction for Qualified Business Income: Section 199A," which is available here, and also see Part IV of Worksheet 12-A on page 56 of the 2018 IRS Publication 535. This part of the QBID logic was not included in pull request #2345 because of an over reliance on TPC's simplified description of QBID logic Even with this addition to QBID, logic there are aspects of QBID logic (multiple sources of pass-through business income in the same filing unit, business income loss carry-forward, etc.) that are not incorporated in Tax-Calculator logic.

Because the TPC QBID example filing units had no qualified dividend or capital gains, the results of the test added in pull request #2343 are unchanged. However, in both the CPS and PUF derived input data sets, there are some filing units that are affected by the new cap, and hence, have a smaller QBID amount, and therefore, have a higher taxable income and individual income tax liability.

[codecov]

Codecov Report

Merging #2348 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #2348   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          13      13           
  Lines        2743    2743           
======================================
  Hits         2743    2743

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a141d93...75fc1bc. Read the comment docs.