EMTR on charitable contributions

[codykallen]

This PR adds the ability to calculate the EMTR on charitable contributions. Since this is negative, this should be interpreted as the effective marginal subsidy on charitable contributions.

Due to limited data on non-itemizers, this MTR should only be considered valid when called on a current law calculator or a reform that reduces or limits itemized deductions. It should also be considered valid only for small values of finite_diff (e.g. the default value of finite_diff of 0.01).

This MTR is useful for estimating the impact of a reform on charitable giving. A reform that limits itemized deductions also reduces the marginal subsidy for charitable contributions, causing individuals to reduce charitable giving.

[codykallen]

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

[codecov-io]

Current coverage is 98.79% (diff: 100%)

Merging #1104 into master will not change coverage

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[martinholmer]

@codykallen, Thanks for your MTR contribution in pull request #1104.

As I remember from the last time we added a variable to the MTR_VALID_VARIABLES list, the addition of a variable automatically generates a new MTR histogram for that new variable in the pufcsv_mtr_expect.txt file. Can you run the full set of unit tests (including those marked requires_pufcsv) to see if the new histogram for variable e19800 looks the way you would expect? If it does (and none of the other MTR results change), then add the new version of the pufcsv_mtr_expect.txt file to this pull request.

[martinholmer]

@codykallen, Thanks for the contribution.