Resolve "Add square sum calculation"

Fix #9 (closed)

Reviewer Checklist

• Is the purpose and scope of the change clear?
• Are eventually defined acceptance criteria met?
• Are the code changes sound and readable?
• Is sufficient documentation provided (comments, user / developer guide)?
• Is the license of eventually newly added dependencies compatible with our license?
• Have been sufficient automated tests provided (code coverage)?
• Are the Git commit(s) (messages) useful?