Streamline logic of create_difference_table utility and improve tbi fuzzing

[martinholmer]

This pull request refactors the create_difference_table utility function and then makes it use the decile_details option added to the add_quantile_table_row_variable function in #1972.

Together these changes produce the following results:

1. The logic of the create_difference_table utility has been considerably simplified and made more accurate.
2. The fuzzing of difference table PUF results in the TaxBrain Interface (tbi) has been strengthened by fuzzing each of the subgroups of the bottom and top deciles (rather than fuzzing just the whole bottom decile and the whole top decile).

[codecov-io]

Codecov Report

Merging #1973 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1973   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          38      38           
  Lines        3783    3756   -27     
======================================
- Hits         3783    3756   -27

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

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0af2a53...b92cacb. Read the comment docs.