Add three ALD amounts to expanded_income

[martinholmer]

This pull request is the second in a series beginning with #1897.
It consists of the three commits 85c63eb and 27acb6b and 98ab493.

This pull request fixes the calculation of expanded_income by correcting the omission of three ALD amounts. The current logic used to compute expanded_income is convoluted: in part it adds to AGI several amounts that were subtracted from income to get AGI. We plan to replace this convoluted logic in a subsequent pull request. But before doing that we are attempting to fix the convoluted logic so that the reasons for and the magnitudes of the errors in the current expanded_income logic are clear.

The three ALD variables and their 2011 PUF amounts (which were missing from expanded_income) are as follows:

Variable	2011 Amt($b)	Description
e03270	25	Self-employed health insurance
e03150	11	IRA contribution
e03300	19	Keogh-SEP contribution

This pull request also fixes the difference table calculation logic so that nan amounts are shown as zeros.

[codecov-io]

Codecov Report

Merging #1899 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1899   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          37      37           
  Lines        3369    3371    +2     
======================================
+ Hits         3369    3371    +2

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

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