DataStructure root node

Currently the root of the `DataStructure` is a special case. We are considering making it more similar to objects like `BaseGroup` to allow it to be referenced in a similar manner. The motivating example was some filters may want to specify the root as a location to place other objects, but with the current setup there is no `DataPath` that refers to the root. Any change should be considered carefully since `DataStructure` affects all filters.