Policy parameter update process

[martinholmer]

Since TCJA became current-law policy, the policy_current_law.json file needs to be updated whenever one of the following things happen:

1. IRS publishes for a new year final forms/instructions containing actual (not projected) parameter values.
2. Tax-Calculator adopts new CPI inflation assumptions in an updated version of the growfactors.csv file.

Before TCJA, only item 1 required an updating of the policy_current_law.json file. But since TCJA, there are several policy parameters that are scheduled to revert to their TCJA-indexed pre-TCJA values in 2026. This aspect of TCJA implies that any change in inflation-rate assumptions will require an updating of values for these reverting parameters in the policy_current_law.json file.

Regardless of whether item 1 or item 2 triggers the policy_current_law.json update, the values through 2026 for each of these reverting parameters can be calculated using the Tax-Calculator/ppp.py script, which was added in pull request #2181.

When item 1 triggers the policy_current_law.json update, the pull request should start with several small commits with each commit adding the last known actual parameter values for parameters on just one IRS form/instructions. And the pull request comment should include links to the final versions of each IRS form and its instructions, which are the source of the values being added to the policy_current_law.json file. After changes associated with each new form/instructions have been committed, the reverting-parameter values through 2026 can be calculated using the (possibly edited) Tax-Calculator/ppp.py script. Also, the value of Policy.LAST_KNOWN_YEAR would need to be incremented.

Note that the exact values of some EITC parameters are not shown in the IRS forms/instructions; get those values from the annual Internal Revenue Bulletin that announces the values of indexed parameters. The 2018 values were announced in this document.