Debt calculator has been added. Please review before approving merge request.

The debt calculator functionality seems to be working. Missing data is checked for and I think that I have implemented all of the error handling that you would expect such as not allowing negative numbers, no numbers with greater than 2 decimal places, interest rate must be between 0% and 100%, and the date can't be on or before current date. If somebody could please clone the repository from this branch and test the functionality, that would be much appreciated. Upon confirming that things are working, you can merge into main yourself or notify me and I will do it. If there are any errors, or anything that you think should be changed, just let me know and I'll see what I can do.

Please note that this code has not been documented yet, but that will be the next step in my process as I wanted to get everything working first and then worry about documentation.