Saver's Credit

[Mshuning]

Fixes #3841

[Mshuning]

@PavelMakarchuk There will be no errors if changing all starting years in every saver's credit parameter files from 2019-01-01 to 2018-01-01.

[Mshuning]

@PavelMakarchuk Should able_contributions be included refer to this?

Amount of the credit
Depending on your adjusted gross income reported on your Form 1040 series return, the amount of the credit is 50%, 20% or 10% of:

contributions you make to a traditional or Roth IRA,
elective salary deferral contributions to a 401(k), 403(b), governmental 457(b), SARSEP, or SIMPLE plan,
voluntary after-tax employee contributions made to a qualified retirement plan (including the federal Thrift Savings Plan) or 403(b) plan,
contributions to a 501(c)(18)(D) plan, or
contributions made to an ABLE account for which you are the designated beneficiary (beginning in 2018).

[MaxGhenis]

@nikhilwoodruff do we need to do anything if renaming an imputed PUF variable?