Make it easier to get frac, weighted_frac as a map

[dbaston]

Adds a frac_as_map argument that makes the frac and weighted_frac operations produce a map instead of an array that must be matched with the values of unique. Only works in Python and for JSON output.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.94%. Comparing base (f45a6df) to head (6ab19c5).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #129      +/-   ##
==========================================
+ Coverage   90.76%   90.94%   +0.18%     
==========================================
  Files          85       85              
  Lines        6196     6287      +91     
  Branches      614      621       +7     
==========================================
+ Hits         5624     5718      +94     
+ Misses        540      537       -3     
  Partials       32       32              

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