Resolve "Add average calculation"

Fixes #11 (closed)

• Is the purpose and scope of the change clear? Yes

• Are eventually defined acceptance criteria met? Tests pass. Tried average of 3 + 4 + 5 = 4, correct.

• Are the code changes sound and readable?

• Is sufficient documentation provided (comments, user / developer guide)? Of course. "We don't need any steenking commentz here anyways"

• Is the license of eventually newly added dependencies compatible with our license? We added no new dependencies

• Have been sufficient automated tests provided (code coverage)? Added two tests. Could use self.assertEqual() instead of raw assert.

• Are the Git commit(s) (messages) useful? Terse, but there's not much to say anyway.

Thanks, gute Arbeit.!!! 👍🏽