Introduce some entropy related functions

[ytdHuang]

Checklist

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Description

In this PR, we introduce the following entropy related functions:

• entropy_linear
• entropy_mutual
• entropy_conditional
• entropy_relative

I also separate the source code file of metrics.jl into entropy.jl and metrics.jl.

Related issues or PRs

implement some functions listed in issue #414

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.29%. Comparing base (7d58e46) to head (df75458).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #416      +/-   ##
==========================================
+ Coverage   94.20%   94.29%   +0.08%     
==========================================
  Files          46       47       +1     
  Lines        2918     2944      +26     
==========================================
+ Hits         2749     2776      +27     
+ Misses        169      168       -1     

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[ytdHuang]

I don't think the significant slowdown in benchmark tracking is related to this PR...

[albertomercurio]

It is the pseudo-inverse benchmarks that apparently varies a lot.