Added tetrahedral_number.rs

[WinterPancake]

Pull Request Template

Description

https://en.wikipedia.org/wiki/Tetrahedral_number

Added a new math algorithm for tetrahedral numbers. Tetrahedral numbers are figurate numbers that represent a tetrahedron.

Type of change

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.

Please make sure that if there is a test that takes too long to run ( > 300ms), you #[ignore] that or
try to optimize your code or make the test easier to run. We have this rule because we have hundreds of
tests to run; If each one of them took 300ms, we would have to wait for a long time.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.52%. Comparing base (99e33d1) to head (9a57953).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #897      +/-   ##
==========================================
+ Coverage   95.32%   95.52%   +0.20%     
==========================================
  Files         319      320       +1     
  Lines       20807    23022    +2215     
==========================================
+ Hits        19834    21992    +2158     
- Misses        973     1030      +57     

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