Add developer script to update TCJA policy parameters

[martinholmer]

This pull request adds a developer-only script that automates the updating of policy parameters that are slated to revert in 2026 to their pre-TCJA-reform values indexed by the TCJA inflation indexing rules. Here is the script's docstring:

Policy parameter projection script, which calculates future policy parameter
values under TCJA and which should be used when the following happens:
(1) the inflation factor values change in growfactors.csv, or
(2) the last known historical values of policy parameters are updated,
    in which case the byear parameter (see below) should be incremented.
USAGE:  $ python ppp.py
OUTPUT: ppp.old -- contains old parameter values in policy_current_law.json
        ppp.new -- contains new parameter values that should now be used

When byear is set to 2018, the script produces results that are, for the most part, within a few dollars of the values in the policy_current_law.json file.

[codecov-io]

Codecov Report

Merging #2181 into master will not change coverage.
The diff coverage is n/a.

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  Coverage    99.9%   99.9%           
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  Files          15      15           
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  Hits         3238    3238           
  Misses          3       3

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