inflation automation

[bodiyang]

This PR aims to implement a new method in Tax-Calculator to automate inflation adjustment each year, following IRS's publication of forms/instructions containing actual (not projected) parameter values

files for the year 2021 as an example
Form 1040 and instructions, Form 1040 (Schedule 1), Form 1040 (Schedule 2), Form 1040 (Schedule 4), Form 1040 (Schedule A) and instructions
Form W-2 and instructions
Form 6251 and instructions
Form 8863 and instructions
Form 8812 and instructions

Previously, this process of inflation parameters replacement is completed manually, as PR #2633

Basic steps to implement the inflation automation:
(1). Collect tax parameters' base year values and make the update in policy_current_law.json
(2). Write a new method to calculate each year's inflation adjusted values, calculated by CPI or chained CPI and then rounded following rounding rules

The calculation process in (2) above is based on:
(a). Tax parameter values' inflation adjustment is calculated by CPI or chained CPI, following $$ ReferenceyearValue = BaseyearValue * {CPI(Referenceyear) \over CPI(Baseyear)}$$
(b). Calculated tax parameter values are then rounded to the near $25, $50 or $100 following the IRS tax parameter rounding rules

[codecov]

Codecov Report

Merging #2661 (4acb1a4) into master (9774b47) will not change coverage.
The diff coverage is n/a.

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##           master    #2661   +/-   ##
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  Coverage   98.54%   98.54%           
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  Files          14       14           
  Lines        2609     2609           
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  Hits         2571     2571           
  Misses         38       38           

Flag	Coverage Δ
unittests	98.54% <ø> (ø)

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[bodiyang]

made a mistake on the selection branch; will create another PR on this issue