Add new EITC policy parameter and associated logic

[martinholmer]

This pull request adds a new policy parameter called _EITC_excess_InvestIncome_rt, which specifies the rate at which the EITC amount is reduced for each dollar of investment income that exceeds the _EITC_InvestIncome_c ceiling. Under current-law, any excess investment income results in EITC ineligibility, so the current-law value of the new parameter is set to 9e99. But this reduction rate could be set as low as zero, in which case investment income in excess of the ceiling would not reduce a filing unit's EITC amount.

The current-law value of this new policy parameter simply parameterizes the EITC reduction logic that previously was hard-coded into Tax-Calculator logic. Hence, the current-law output generated by Tax-Calculator is unchanged. What is gained is the ability to specify alternatives to current law.

[codecov-io]

Codecov Report

Merging #2164 into master will not change coverage.
The diff coverage is n/a.

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##           master   #2164   +/-   ##
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  Coverage    99.9%   99.9%           
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  Files          15      15           
  Lines        3241    3241           
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  Hits         3238    3238           
  Misses          3       3

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