Allow for alternative policy baselines

[jdebacker]

This PR updates OG-USA to allow one to specify the baseline individual income tax policy so that alternative IIT baseline (i.e., alternatives to the Tax-Calculator current law baseline) can be used in OG-USA.

An example script that implements a current policy baseline is added to illustrate usage.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 75.35%. Comparing base (e4655db) to head (8229008).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #106      +/-   ##
==========================================
- Coverage   75.41%   75.35%   -0.06%     
==========================================
  Files          11       11              
  Lines         850      852       +2     
==========================================
+ Hits          641      642       +1     
- Misses        209      210       +1     

Flag	Coverage Δ
unittests	75.35% <88.88%> (-0.06%)	⬇️

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Files	Coverage Δ
ogusa/calibrate.py	37.41% <ø> (ø)
ogusa/get_micro_data.py	96.82% <88.88%> (-1.54%)	⬇️

[jdebacker]

@rickecon Are you ok with merging this? It's needed to work with recently merged changes to OG-Core (PR #927).