Add expanded_income and aftertax_income to distribution table

[martinholmer]

This pull request adds two new columns to the distribution table as requested in issue #1374.
The pull request also removes several redundant tests in the test_calculate.py file.

There is no changes in tax-calculating logic or results. There are only two more columns in each table produced by the create_distribution_table utility function.

[codecov-io]

Codecov Report

Merging #1602 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1602   +/-   ##
======================================
  Coverage     100%    100%           
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  Files          37      37           
  Lines        2767    2767           
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  Hits         2767    2767

Impacted Files	Coverage Δ
taxcalc/utils.py	100% <ø> (ø)	⬆️

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[MattHJensen]

Looks great. Thanks @martinholmer.