bug fix and additional checks

[FarzeenKist]

Improvements and Fixes

1. Implemented the IERC721Receiver for best practices when dealing with transfers of NFTs to a foreign contract
2. Renamed mulDiv into calculateInterestAmount and switched it to be a public function to allow users to have the functionality to calculate the estimated amount they can make off or pay off a loan.
3. There was also a bug in mulDiv(you can use 250,10, and 100 to see the issue). I've gone ahead and fixed that and also simplified the logic of calculating the interest while also adding a check to prevent loan amount below 1 CUSD as a really low amount could lead to issues where the interest amount would be lower than expected
4. removed z as an argument for mulDiv as it was constant and unnecessary
5. Added the modifier exists to prevent access to loans that have not yet been created as this could lead to unexpected behaviors/issues (users could lendMoney function)
6. Added checks for _nft and _loanDuration to prevent unexpected behaviors/issues as zero being entered as the _loanDuration could lead to issues such as the loan instantly being foreclosed and the borrower being able to take the NFT being used as collateral

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