Include setax and ptax_amc in income tax rather than in payroll tax

[martinholmer]

Fixes #2843 bug by including both the self-employment tax (setax) and the additional Medicare tax (ptax_amc) in othertaxes, and therefore, in the iitax variable (rather than including them both in the payrolltax variable).

Note that the potentially-confusing variable name for the additional Medicare tax (ptax_amc) has been retained in order to avoid breaking backwards compatibility.

Most of the test changes in this PR involve changing expected iitax and payrolltax values to correspond to the values generated by the new version of Tax-Calculator in this PR. However, test_itax_compare in the taxcalc/tests/test_compare.py module, is different in this respect. Its expected values are published IRS/SOI income tax statistics for 2015. Before the changes in this PR, the aggregate iitax generated by the model using the puf.csv input data was 1406.456 billion dollars, which was about 3.5 percent below the IRS/SOI amount of 1457.891 billion dollars. Using the changes in this PR, the model now generates 1462.385 billion dollars, which is about 0.3% above the IRS/SOI amount of 1457.891 billion dollars.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.42%. Comparing base (404ac22) to head (c15b063).
Report is 2 commits behind head on master.

Additional details and impacted files

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##           master    #2844   +/-   ##
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  Coverage   99.42%   99.42%           
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  Files          13       13           
  Lines        2619     2619           
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  Hits         2604     2604           
  Misses         15       15           

Flag	Coverage Δ
unittests	99.42% <100.00%> (ø)

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Files with missing lines	Coverage Δ
taxcalc/__init__.py	100.00% <100.00%> (ø)