Add NIIT_PT_taxed policy parameter and NIIT logic

[martinholmer]

This pull request adds the ability to simulate a Clinton income tax reform provision: the inclusion of partnership and S-corporation pass-through income in the base of the Net Investment Income Tax. A recent TPC report on page 4 describes this reform provision as follows:

Clinton proposes “rationalizing” the 3.8 percent net investment Income tax (NIIT)
imposed by the Affordable Care Act on investment income for filers with incomes
above $200,000 ($250,000 for married filers).  Clinton would broaden the tax base
of the NIIT to include all pass-through business income not subject to the self-
employment payroll (SECA) tax.  Currently, some pass-through income is not subject
to either SECA or the NIIT.

This pull request does not alter tax-calculating logic under current-law policy, but does add the ability to simulate this reform provision. Using this pull request produces a static income tax revenue increase for 2018 that is consistent with the corresponding TPC estimate, which is combined together with the estimated effects of several changes in capital-gains-realizations rules. So, "consistent" means that the Tax-Calculator estimate for 2018 is less than the combined TPC estimate for 2018. Details are below for those interested.

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

$ cp ../tax-calculator-data/puf.csv .

$ python inctax.py puf.csv 2018 --blowup --weights 
You loaded data for 2009.
Your data have been extrapolated to 2018.

$ cat ptniit.json 
{
    // include partnership and S-corp income (e26270) in NIITTbase in 2017+
    "_NIIT_PT_taxed":{"2017": [1]}
}

$ python inctax.py puf.csv 2018 --blowup --weights --reform ptniit.json
You loaded data for 2009.
Your data have been extrapolated to 2018.

$ ls puf-18*
puf-18.out-inctax       puf-18.out-inctax-ptniit

$ awk '{r+=$4*$29}END{print r}' puf-18.out-inctax
1.61649e+12

$ awk '{r+=$4*$29}END{print r}' puf-18.out-inctax-ptniit
1.6322e+12

2018 T-C estimate of reform's effect on inctax revenue is $15.71 billion.

The TPC 2018 estimate for this reform are combined together with the estimate
of the impact of several capital-gains-realization reforms; the combined estimate
is $21.6 billion for 2018. (TPC report dated 18-Oct-2016, page 8)

[codecov-io]

Current coverage is 98.02% (diff: 100%)

Merging #1012 into master will decrease coverage by 0.24%

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[MattHJensen]

This LGTM. Thanks @martinholmer!