Clarify the three components of the _payrolltax total

[martinholmer]

This pull request renames one existing variable and adds a new intermediate variable so that the three components of the total _payrolltax variable are more clear. After these changes the components of _payrolltax are named as follows:

ptax_was : FICA tax on wages & salaries (half is employer share)
ptax_sey : Self-Employment Tax (half is "employer" share)
ptax_amc : Additional Medicare Tax on high earnings (there is no employer share)

So, using these new/renamed ptax_* variables, the employee share of the total _payrolltax is:

employee_share = 0.5 * ptax_was + 0.5 * ptax_sey + ptax_amc

The changes in this pull request do not alter any tax-calculating logic, test results, or reform comparisons.

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

[codecov-io]

Current coverage is 98.13% (diff: 100%)

Merging #909 into master will not change coverage

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