Revise tax credit calculations for children and other dependents

[martinholmer]

This pull request updates the calculation of the nonrefundable child tax credit (CTC) and other dependent tax credit (ODTC), the sum of which is entered on line 12a of 2018 Form 1040, and updates the calculation of the refundable additional child tax credit (ACTC), which is entered on line 17b of 2018 Form 1040. These changes are being made because of the problems discussed in open issue #2197 and in merged pull request #2205.

After commit f47e16f, which rewrites the ChildDepTaxCredit function, the code in this pull request still produces the same CTC amounts and the same ACTC amounts for 2017 as on the master branch, which is important because those values have been validated against TAXSIM-27. However, there are a number of 2018 cases for which there are differences with TAXSIM-27 when using the commit f47e16f code.

After commit b99acf1, which rewrites the AdditionalCTC function, the code in this pull request still produces the same CTC amounts and the same ACTC amounts for 2017 as on the master branch, which is important because those values have been validated against TAXSIM-27. However, there are still a number of 2018 cases for which there are differences with TAXSIM-27 when using the commit b99acf1 code.

Future commits will attempt to eliminate the remaining 2018 differences. But before doing this we need 2018 values for the policy parameters, which are pending in pull request #2212.

[codecov-io]

Codecov Report

Merging #2211 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #2211   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          12      12           
  Lines        2948    2948           
======================================
  Hits         2948    2948

Impacted Files	Coverage Δ
taxcalc/calculator.py	100% <100%> (ø)	⬆️

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[martinholmer]

As discussed in issue #2197, beginning in 2018 the Tax-Calculator policy parameters for the CTC and the ACTC are, at the very least, very confusing and misleading. The logic changes in this pull request leave the CTC-related and ACTC-related policy parameter names unchanged. But @Peter-Metz is correct in saying that we need a comprehensive review of the names. Discussing how to improve the the names will be part of a future issue and follow-up pull request.

@MattHJensen @Peter-Metz @hdoupe