Enables having more then one subnft of a given type

[joshuahannan]

Description

Changes the return type of getAvailableSubNFTS() to have an array so that there can be more than one NFT of a given type.

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For contributor use:

• [x] Targeted PR against master branch
• [x] Code follows the standards mentioned here.
• [x] Re-reviewed Files changed in the Github PR explorer