Add Calculator ce_aftertax_income method and make corresponding utility low-level

[martinholmer]

This pull request adds a ce_aftertax_income method to the Calculator class, leaving the low-level details in the corresponding ce_aftertax_expanded_income utility function. This pull request is part of the process of finishing the object-oriented design of Tax-Calculator described in issue #1720.

[codecov-io]

Codecov Report

Merging #1723 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1723   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          37      37           
  Lines        2956    2961    +5     
======================================
+ Hits         2956    2961    +5

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

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[MattHJensen]

@martinholmer, this seems right, but it doesn't look like our current testing allows us to know whether the refactoring changes results. Do you think it would be worthwhile to augment the testing?

[martinholmer]

@MattHJensen said:

[PR #1723] seems right, but it doesn't look like our current testing allows us to know whether the refactoring changes results. Do you think it would be worthwhile to augment the testing?

Yes. Good suggestion to strengthen the testing.

[MattHJensen]

Thanks @martinholmer. The new testing looks great.