Staked TokenIDs stored on-chain

Added functionality for the Staked Token IDs to be stored on-chain and be return as an array by the userStakeInfo() function to simplify UI integration of the Smart Contract.

Added NatSpec.

Re-written tests(tests might fail or succed on multiple runs without changes because of the way brownie interacts with Ganache, if you want more precise tests I'd recommend working with HardHat).

Initial release

The first version of the ERC721 Staking Smart Contract library