User input validation & docstring update

[sdthinlay]

Type of change

• [ ] Added new project
• [ ] Bug fix
• [x] New features
• [x] Enhanced documentation

Changes proposed in this pull request

• Project name: Loan Calculator

• Short description: Initially the game failed to evaluate invalid user input so with the updated code the user input is validated, test case is also added with docstring update.

Checklist

• [x] The code is Python 3 compatible
• [x] I have added relevant comments in my file to make code understandable
• [x] I have added my project in a folder which includes all the necessary files

[github-actions[bot]]

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[Mrinank-Bhowmick]

Please maintain project directory structure