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.

Yes I think it's best this way, even more so, because a LinkedScene can be loaded as a SceneCache (not expanding links) and the hashes should be different in the two scenarios, at least the hierarchy hash.