Add exact calculation logic to CTC_new function

[martinholmer]

Add exact, stair-step phase-out logic when exact is one in the CTC_new (i.e., ARPA CTC addition) function. This change affects tax results only when CTC_new is active (which is only during 2021 under current-law policy) and when the exact_calculations argument of the Records class constructor has been specified to be True.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.42%. Comparing base (9a8cbfd) to head (3779d50).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2769   +/-   ##
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  Coverage   99.42%   99.42%           
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  Files          13       13           
  Lines        2610     2610           
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  Hits         2595     2595           
  Misses         15       15           

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

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