feat: Added NPV, Compound Interest and Payback Period Calculation to 'financial' module

[showmyth]

Pull Request Template

Description

Added 3 algorithms namely,

1. NPV (Net Present Value): Tells us how much a series of payments is worth in current times.
2. Compound Interest: The interest calculated on the initial principal amount as well as the accumulated interest from previous periods.
3. Payback Period: Tells us how long it will take to recover the initial investment using the project's cash flows.

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

❌ Patch coverage is 97.56098% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.35%. Comparing base (379b934) to head (fa80fc4).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/navigation/rhumbline.rs	95.65%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #935      +/-   ##
==========================================
+ Coverage   95.34%   95.35%   +0.01%     
==========================================
  Files         320      324       +4     
  Lines       20970    21093     +123     
==========================================
+ Hits        19994    20114     +120     
- Misses        976      979       +3     

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