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 would prefer a more robust method, of course. When do you anticipate getting to the more complete hash method with IndexedIO support? If you want to merge as-is perhaps you could you at least add a comment outlining the vulnerability?