Streamline dependent-credit logic and update documentation

[martinholmer]

This pull request streamlines dependent-credit logic by eliminating a redundant policy parameter (_DependentCredit_c) and simplifying the dependent credit phase-out calculations. It also updates the documentation of the policy parameters related to the dependent credit and adds documentation to the records_variables.json file for the dep_credit calculated variable.

Simplifying the dependent-credit phase-out logic changes the 2018 CPS estimate of Child Tax Credit plus Dependent Credit from $132.138 billion to $132.397 billion, which is an increase of one-fifth of one percent.

@codykallen and @MattHJensen and others with an interest, please review the changes in this pull request to make sure that the new code and documentation correctly represent dependent-credit phase-out logic.

[codecov-io]

Codecov Report

Merging #1937 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1937   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          38      38           
  Lines        3629    3629           
======================================
  Hits         3629    3629

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

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