Financials (#5585)

* feat: creates math calculations for financials

* refactor: make pull request items requirements

* refactor: provides type hint for parameters

* refactor: applies code review suggestions

* refactor: adds more examples tests

* refactor: throws ValueError instead of Exception

* refactor: fix formatting

* refactor: fix formatting

* Update interest.py

* Update and rename financials/ABOUT.md to financial/ABOUT.md

* Rename financials/__init__.py to financial/__init__.py

* Rename financials/interest.py to financial/interest.py

* https://www.investopedia.com

* Update __init__.py

* pre-commit: Disable end-of-file-fixer

* Revert change to pre-commit

* Update __init__.py

* __init__.py

Co-authored-by: Christian Clauss <cclauss@me.com>
Co-authored-by: John Law <johnlaw.po@gmail.com>