Revise diagnostic table logic making it easier to create when assuming behavioral responses

[martinholmer]

This pull request answers the question posed by @evtedeschi3 in issue #2125. By moving some of the low-level logic of creating multi-year diagnostic tables from the Calculator class to the create_diagnostic_table utility function, it is now easier to create a multi-year diagnostic table when assuming behavioral responses to a policy reform. The new approach, which requires Tax-Calculator 0.23.2 or higher, is illustrated in the expanded Cookbook recipe 2.

The Calculator method diagnostic_table method works exactly as before, so there is no API change there.
However, the signature of the create_diagnostic_table utility function has been changed, so that is a minor API change. Given that most people are using the Calculator method rather than the low-level utility function, we are calling this pull request a "New Feature" rather than an "API Change".

[codecov-io]

Codecov Report

Merging #2126 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2126      +/-   ##
==========================================
+ Coverage   99.91%   99.91%   +<.01%     
==========================================
  Files          16       16              
  Lines        3512     3518       +6     
==========================================
+ Hits         3509     3515       +6     
  Misses          3        3

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

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