Alternative baselines

[jdebacker]

This PR updates CCC to allow for alternative individual income tax baselines.

A JSON file that parameterizes TCJA permanence is added -- as is a notebook illustrating how to use this file.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 99.82%. Comparing base (2a1b498) to head (dbc2e7b).

Additional details and impacted files

@@             Coverage Diff             @@
##            master     #388      +/-   ##
===========================================
- Coverage   100.00%   99.82%   -0.18%     
===========================================
  Files           11       11              
  Lines         1139     1143       +4     
===========================================
+ Hits          1139     1141       +2     
- Misses           0        2       +2     

Flag	Coverage Δ
unittests	99.82% <60.00%> (-0.18%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
ccc/parameters.py	100.00% <100.00%> (ø)
ccc/get_taxcalc_rates.py	96.61% <50.00%> (-3.39%)	⬇️