[ENH] KCD and Bregman conditional 2-sample tests

[adam2392]

Closes: #15
Follow-up to #19

Changes proposed in this pull request:

• Adds kcd and bremgan test along w/ unit-tests and documentation update
• As a result of [ENH] Faster and more flexible code, and code sharing for kernel tests #19, the code is entirely self-contained and leverages the kernel functions that are shared w/ the kci test.

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[codecov]

Codecov Report

Merging #21 (03fb9d0) into main (256d8c9) will increase coverage by 6.88%.
The diff coverage is 98.49%.

@@            Coverage Diff             @@
##             main      #21      +/-   ##
==========================================
+ Coverage   82.83%   89.71%   +6.88%     
==========================================
  Files           9       12       +3     
  Lines         402      535     +133     
  Branches       72       83      +11     
==========================================
+ Hits          333      480     +147     
+ Misses         49       35      -14     
  Partials       20       20              

Files Changed	Coverage Δ
pywhy_stats/conditional_ksample/bregman.py	95.00% <95.00%> (ø)
pywhy_stats/conditional_ksample/base_propensity.py	100.00% <100.00%> (ø)
pywhy_stats/conditional_ksample/kcd.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[bloebp]

Looks great! Only some minor comments and clarification questions.