Add jeffreys

[koheiw]

Add "jeffreys" to dist() for Jeffreys divergence for #31. It is a symmetric version of Kullback-Leibluer divergence.

See Eq. 15 in https://link.springer.com/article/10.1007/s10044-015-0453-7.

[codecov[bot]]

Codecov Report

Merging #32 (b203caa) into master (e1d0090) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
+ Coverage   99.46%   99.47%   +0.01%     
==========================================
  Files           4        4              
  Lines         371      382      +11     
==========================================
+ Hits          369      380      +11     
  Misses          2        2              

Impacted Files	Coverage Δ
R/proxy.R	99.07% <100.00%> (+<0.01%)	:arrow_up:
src/pair.cpp	100.00% <100.00%> (ø)

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[koheiw]

@rcannood Is it possible for you to review this PR. The changes are small and straightforward.

[koheiw]

Thank for reviewing. The name of the person is actually Harold Jeffreys. The authors of the paper misspelled the name.