Fix stat_dataframe function nested in create_distribution_table utility function

[martinholmer]

This pull request fixes a Tax-Calculator bug that was diagnosed by @hdoupe in PolicyBrain issue 794. His ingenious scripts determined that the bug was in Tax-Calculator (because the dictionary tbi results were different from the dataframe tbi results) and that those differences occurred only in distribution table results, not in difference table results or aggregate results. Thanks @hdoupe for the extremely helpful bug report!

Using Hank's script (that compares tbi distribution table results returned as a dataframe with tbi distribution table results returned as a dictionary), I used standard bisection methods to determine that there were no differences in release 0.13.2 and earlier and differences in the next release, which was numbered 0.14.0, and in subsequent releases. Then reviewing commits made during the few weeks between 0.13.2 (2017-11-17) and 0.14.0 (2017-12-11), revealed commit aef4b20 that introduced a nested function called stat_dataframe into the create_distribution_table utility function. Given @hdoupe's diagnosis (the numbers in the dictionary were correct but they were not associated with the correct variable), the source of the bug was obvious once attention was focused on the stat_dataframe function.

The changes in this pull request leave all the unit and validation tests that were passing before this pull request still passing. There are no changes in tax calculating logic.

My guess is that this (now fixed) bug affected only TaxBrain, not other clients of Tax-Calculator.

[codecov-io]

Codecov Report

Merging #1834 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1834   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          37      37           
  Lines        3098    3100    +2     
======================================
+ Hits         3098    3100    +2

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

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[hdoupe]

@martinholmer This looks good to me. Thanks for quickly isolating and fixing this bug. When will Tax-Calculator 0.15.0 be released?

PolicyBrain will require a few other updates to catch up with Tax-Calculator 0.15.0. It will probably be a day or two before we can do a PolicyBrain release with the update.

[martinholmer]

@hdoupe asked in pull request #1834:

When will Tax-Calculator 0.15.0 be released?

PolicyBrain will require a few other updates to catch up with Tax-Calculator 0.15.0. It will probably be a day or two before we can do a PolicyBrain release with the update.

Today. Even though @andersonfrailey is making excellent progress on pull request #1719, I think its better to get 0.15.0 out now so that TaxBrain can use it asap. And then in a few more days, we'll merge #1719 and release 0.15.1 that contains that enhancement.

[hdoupe]

Great, thanks @martinholmer