Adding hash function to SceneInterface

I've added a hash function with an enum that computes different kinds of hashes (for Object, Transform, child names, etc) for a given point in time.

This will allow better use of caching mechanisms when loading scenes as well as use of instances in engines such as Gaffer.

Ok, as long we all agree with the interpretation of hierarchy hash. Makes sense to me. Will affect the LinkedScene implementation as it is.