Implemented quick select iteratively, and an algorithm to make a deep copy of an undirected graph. (C++)

[alexdorfman81]

Fixes issues: https://github.com/OpenGenus/cosmos/issues/4604 https://github.com/OpenGenus/cosmos/issues/6218

Changes: Implemented the quick select algorithm iteratively with a very detailed explanation. (C++) Implemented a simple and time-efficient algorithm to make a deep copy of a graph given a reference to a node. (C++)