Address issue 20

[koheiw]

Add use_nan argument for #20. simil(method = "correlation", use_nan = TRUE) will return NaN for vectors without variance.

I am happy to change the name of the argument if @rcannood can has a better idea.

[codecov]

Codecov Report

Merging #21 (750c5b9) into master (fde2242) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
+ Coverage   99.11%   99.15%   +0.04%     
==========================================
  Files           4        4              
  Lines         338      355      +17     
==========================================
+ Hits          335      352      +17     
  Misses          3        3              

Impacted Files	Coverage Δ
R/proxy.R	99.04% <100.00%> (+0.04%)	⬆️
src/linear.cpp	98.78% <100.00%> (+0.09%)	⬆️
src/pair.cpp	99.25% <100.00%> (+0.03%)	⬆️

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[rcannood]

Brilliant! おつかれさまでした！

I added one more commit which renames 'nan' to 'use_nan' to avoid naming conflicts between the parameter 'nan' and the function 'std::nan', just in case.

[koheiw]

@rcannood thank you for reviewing. I will update CRAN soon.