Do not extended Statistics.cor/Statistics.cov

[giordano]

Using the same functions as the Statistics ones is a pun, but the functions do different things, in particular they return scalar values, while the method defined here return matrices and make it impossible to do the same thing as the Statistics methods.

Also, adding the Statistics package as dependency with the sole purpose of extending methods of functions, which could have been done with an extension, increases loading time by a factor of 40.

Ref: f1d6e89#r115384906. CC: @LukasACH

[codecov]

Codecov Report

Merging #148 (fe65218) into master (628aad2) will not change coverage.
The diff coverage is 100.00%.

❗ Current head fe65218 differs from pull request most recent head 2d41312. Consider uploading reports for the commit 2d41312 to get more accurate results

@@           Coverage Diff           @@
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  Coverage   96.91%   96.91%           
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  Files           9        9           
  Lines         616      616           
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Impacted Files	Coverage Δ
src/Measurements.jl	77.41% <ø> (ø)
src/utils.jl	100.00% <100.00%> (ø)

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[LukasACH]

I based my decision to extend Statistics on Distributions.jl which extends Statistics as well, and returns matrices for the cor and cov functions. But if it increases load times that much, it indeed does not make much sense.

[giordano]

I based my decision to extend Statistics on Distributions.jl which extends Statistics as well, and returns matrices for the cor and cov functions

Distributions are their own entirely different datatype, Measurements are Numbers, and the correlation/covariance of vectors of numbers are already well-defined.