Threshold for self-employment income exempt from SECA taxes

[jdebacker]

Adds parameters for threshold on self-employment income subject to SECA taxes, addressing Issue #2656.

[jdebacker]

The changes here results in some differences in the comparison files that show increased differences between T-C and SOI tabulations when using CPS data (also some small diffs with the PUF). For example, the "expected" file before this PR shows:

TABLE for ITAX
AGI category           	     T-C	     SOI	 %diff
[-9e+99, 1)            	  -0.020	   0.242	-108.0
[1, 5000)              	  -1.630	   0.041	-4084.5
[5000, 10000)          	  -7.181	   0.368	-2051.4

After the changes proposed here, this becomes:

TABLE for ITAX
AGI category           	     T-C	     SOI	 %diff
[-9e+99, 1)            	  -0.020	   0.242	-108.0
[1, 5000)              	  -1.630	   0.041	-4084.7
[5000, 10000)          	  -7.181	   0.368	-2051.4

I'm not sure if this is something to be concerned about.

[codecov]

Codecov Report

Merging #2659 (8969262) into master (9774b47) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2659   +/-   ##
=======================================
  Coverage   98.54%   98.54%           
=======================================
  Files          14       14           
  Lines        2609     2609           
=======================================
  Hits         2571     2571           
  Misses         38       38           

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

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[jdebacker]

@MaxGhenis Does this PR look like a good response to Issue #2656 to you?

[MaxGhenis]

LGTM thanks @jdebacker!

[jdebacker]

@MattHJensen can you review this PR and merge if it looks good to you? Thanks!

[MattHJensen]

Thanks for reviewing with me on the TC meeting today. Merging.