Add Calculator.pch_graph method and associated utility functions

[martinholmer]

This pull request adds a new graph method to the Calculator class. The new pch_graph method generates a graph in which the percentage change in after-tax expanded income (cause by a policy reform) is plotted for each baseline expanded-income percentile except for percentiles that contain filing units with negative or zero baseline expanded income. This graph, therefore, avoids any distortions caused by some filing units having negative or zero baseline expanded income. These filing units are in the bottom several percentiles, but those percentiles are not shown in the graph.

This pull request resolves the issue discussed in #1888 by using this graph in the tc --graphs output (instead of the decile or quintile graphs that include filing units with negative income) and by making several documentation changes that clarify whether a table or graph includes filing units with negative income.

Here is an example of what the new graph looks like. The following graph is generated using puf.csv data and the reform from the basic recipe in the Cookbook of Tested Recipes for Python Programming with Tax-Calculator:

// Raise personal exemption and raise regular tax rates in top three brackets
// from (0.32, 0.35, 0.37) to (0.36, 0.39, 0.41) beginning in 2020.
{
    "policy": {
	// raise personal exemption amount and index in subsequent years
	"_II_em": {"2020": [1000]},
	// raise tax rates in the top three brackets
	// ... raise non-AMT rates on non-pass-through income
	"_II_rt5": {"2020": [0.36]},
	"_II_rt6": {"2020": [0.39]},
	"_II_rt7": {"2020": [0.41]},
	// ... raise non-AMT rates on pass-through income
	"_PT_rt5": {"2020": [0.36]},
	"_PT_rt6": {"2020": [0.39]},
	"_PT_rt7": {"2020": [0.41]}
    }
}

Here is the new graph for this example:

[codecov-io]

Codecov Report

Merging #1890 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1890   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          37      37           
  Lines        3301    3362   +61     
======================================
+ Hits         3301    3362   +61

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

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[martinholmer]

Given the extensive discussion in issue #1888, it would seem that the graph in pull request #1890 is consistent with a range of future changes in the handling of filing units with non-positive expanded_income in the construction of tables and graphs. Given that, I plan to merge pull request #1890 at the end of the week on Friday, March 2nd.