Added Hill Cypher

[MegRCooper]

Added a simple Hill cypher that uses a 3x3 Matrix. provides user option for both encryption and decryption. Marked with "?" on the TODO .

Short description of what this resolves:

Added feature from TODO list

Changes proposed in this pull request:

Status (please check what you already did):

• [X] made PR ready for code review
• [ ] added some tests for the functionality
• [ ] updated the README with example

Fixes: #