Fix top-decile-detail logic in create_distribution_table utility function

[martinholmer]

This pull request fixes a bug in create_distribution_table logic that generates information for subgroups of the top income decile. This bug was discovered while preparing a forthcoming pull request. The bug was caused by the initial design of the create_distribution_table utility function, which did work until top-decile-detail logic was first added. The solution was to redesign create_distribution_table logic to make it be more like the logic in the create_difference_table utility function. This bug fix leaves the basic ten-decile and All results unchanged.

This bug is in the current 0.13.2 release of Tax-Calculator, but it has limited impact because TaxBrain does not (yet?) show the top-decile subgroup statistics for the distribution table (or for the difference table, for that matter).

[codecov-io]

Codecov Report

Merging #1712 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1712   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          37      37           
  Lines        2923    2927    +4     
======================================
+ Hits         2923    2927    +4

Impacted Files	Coverage Δ
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 6d8f46f...10b9060. Read the comment docs.