Add logic for refundable credit claiming behavior

[martinholmer]

This pull request represents step 1 of the two-step development plan described in issue #3080.

See the detailed logic of EITC claiming in section E logic at the end of the EITC function in the calcfunctions.py module.

See the detailed logic of ACTC claiming at the end of the AdditionalCTC function in the calcfunctions.py module.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (8bb3c13) to head (cabcd14).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #3079   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines         2659      2661    +2     
=========================================
+ Hits          2659      2661    +2     

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

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

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