Feature/parent node derivation (#35)

[q9f]

Resolved conflict on #35

Added this method with the purpose of learning and test that is possible to get a parent node with a child's private key and a parent public key.

Maybe no one will use it and that is why it didn't exist in the first place. Or It could be some security concern to provide this method?

I wanted to see why and how hardening works.

Co-authored-by: aldo [email protected]