Add _BE_subinc_wrt_earnings Behavior parameter

[martinholmer]

This pull request leaves the existing usage and logic of the Behavior class unchanged.

It adds a new optional kind of behavioral response in which the income and substitution elasticities are applied to earnings instead of taxable income. Note: pull request #1556 builds on these changes.

One of the existing Behavior tests has been modified in order to maintain complete code coverage.

@MattHJensen @feenberg @Amy-Xu @andersonfrailey @hdoupe @GoFroggyRun @codykallen

[codecov-io]

Codecov Report

Merging #1554 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1554   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          37      37           
  Lines        2593    2603   +10     
======================================
+ Hits         2593    2603   +10

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

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