Allow compound reforms when using tc tool --reform option

[martinholmer]

This pull request enhances the existing tc --reform REFORM option so that it can handle a compound reform.
A compound reform is a reform that is composed of two (or more) individual reforms. For an example of a compound reform, see the section entitled A Round-Trip Compound Reform at the bottom of this FAQ.

So, for example, if a user has specified reformA.json and reformB.json and is interested in analyzing the joint effect of the two reforms (that is, first implementing reformA relative to current-law policy and then implementing reformB relative to reformA policy), the user simply executes a command like this:

$ tc cps.csv 2020 --reform reformA.json+reformB.json

The above command will generate two output files with the following names:

cps-20-reformA+reformB-#-doc.text    <==== compound reform documentation
cps-20-reformA+reformB-#.csv         <==== minimal output file

As one would expect, using this new feature produces the following results:

$ cp ../tax-calculator/taxcalc/reforms/2017_law.json .
$ cp ../tax-calculator/taxcalc/reforms/TCJA_Reconciliation.json ./TCJA.json
$ ls *json
2017_law.json   TCJA.json

$ tc cps.csv 2020 --tables
You loaded data for 2014.
Tax-Calculator startup automatically extrapolated your data to 2020.
$ ls cps-20-*
cps-20-#-#-doc.text	cps-20-#-#-tab.text	cps-20-#-#.csv

$ tc cps.csv 2020 --tables --reform 2017_law.json+TCJA.json
You loaded data for 2014.
Tax-Calculator startup automatically extrapolated your data to 2020.
$ ls cps-20-*
cps-20-#-#-doc.text		cps-20-2017_law+TCJA-#-doc.text
cps-20-#-#-tab.text		cps-20-2017_law+TCJA-#-tab.text
cps-20-#-#.csv			cps-20-2017_law+TCJA-#.csv

$ diff  cps-20-#-#-tab.text  cps-20-2017_law+TCJA-#-tab.text 
$

As with the Python programming in #1830, the round-trip compound reform (of implementing 2017_law.json and then TCJA.json) produces exactly the same results as does current-law policy.

The need for this enhancement was discussed as part of merged pull request 1803 beginning with this comment.

[codecov-io]

Codecov Report

Merging #1842 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1842   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          37      37           
  Lines        3103    3141   +38     
======================================
+ Hits         3103    3141   +38

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

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