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.

I don't understand why the transform isn't a part of the hash - can you explain? In Gaffer at least, it is very common for a leaf location to have a transform.