chore: Update AVL Tree

[ritk20]

Description of Change

Updated AVL Tree data structure using modern C++ STL library features like std::unique_ptr and implemented Object Oriented structure. Also added additional tests and updated existing tests to use assert.

Checklist

• [x] Added description of change
• [x] Added file name matches File name guidelines
• [x] Added tests and example, test must pass
• [x] Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
• [x] Relevant documentation/comments is changed or added
• [x] PR title follows semantic commit guidelines
• [x] Search previous suggestions before making a new one, as yours may be a duplicate.
• [x] I acknowledge that all my contributions will be made under the project's license.

Notes: Completed the following TODO

/**
 * \todo update code to use C++ STL library features and OO structure
 * \warning This program is a poor implementation and does not utilize any of
 * the C++ STL features.
 * /