DataStructure root node

[JDuffeyBQ]

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.