Add new functions to Math MK2 node: mean and gcd (greatest common divisor)

Created by: DolphinDream

Addressed problem description

Add some missing functions: arithmetic mean: (x + y) /2 and gdc (greatest common divisor).

Preflight checklist

• Code changes complete.
• Code documentation complete.
• Documentation for users complete (or not required, if user never sees these changes).
• Manual testing done.
• Unit-tests implemented.
• Ready for merge.