Add Net Investment Income Tax

[nikhilwoodruff]

New variable: Net Investment Income Tax

• Label field added
• Documentation field added
• Unit field added
• Default value field added if relevant
• Variable name follows conventions
• Unit test(s) added
• Integration test(s) added if relevant

What's changed

Implements the parameters and variable from NetInvIncTax in taxcalc.

[codecov]

Codecov Report

Merging #343 (b05d4ae) into master (eed9026) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #343      +/-   ##
==========================================
+ Coverage   93.93%   93.95%   +0.01%     
==========================================
  Files          46       46              
  Lines        3117     3126       +9     
  Branches      436      436              
==========================================
+ Hits         2928     2937       +9     
  Misses        165      165              
  Partials       24       24              

Impacted Files	Coverage Δ
openfisca_us/variables/irs/outputs.py	97.27% <100.00%> (+0.03%)	⬆️

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[nikhilwoodruff]

@MaxGhenis I've added some unit tests testing the NIIT value against tax-calc results (and added the script to this PR)