Added geometric mean to averages.rs

[Marmaa0]

Description

Added a geometric mean to the averages.rs file, whilst mean, median and mode are certainly more standard than the geometric mean, it is often used within finance, and a valuable average when confronted with sequences which have a multiplicative nature. I've tried to follow the same style as the previous averages, I'm very new to rust and programming in general so I apologise in advance if I have made some obvious or glaring errors.

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

Checklist:

• I ran bellow commands using the latest version of rust nightly.
• I ran cargo clippy --all -- -D warnings just before my last commit and fixed any issue that was found.
• I ran cargo fmt just before my last commit.
• I ran cargo test just before my last commit and all tests passed.
• I added my algorithm to the corresponding mod.rs file within its own folder, and in any parent folder(s).
• I added my algorithm to DIRECTORY.md with the correct link.
• I checked COUNTRIBUTING.md and my code follows its guidelines.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.45%. Comparing base (f66f836) to head (28b198d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #846      +/-   ##
==========================================
+ Coverage   95.44%   95.45%   +0.01%     
==========================================
  Files         316      316              
  Lines       22750    22806      +56     
==========================================
+ Hits        21713    21769      +56     
  Misses       1037     1037              

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