Inheritance of the NXroot class from NXobject

[rayosborn]

At the recent NIAC, it was officially approved to allow base classes to inherit from the NXobject base class, in order to formalize sections of the documentation that allow certain classes, such as NXnote and NXcollection, to be added to any group. This is an important improvement in formalizing the status of these special groups, but it leads to a problem with the NXroot base class. At the moment, this only allows NXentry groups. With inheritance, suddenly a whole raft of additional groups, potentially including a number with nameType=partial, e.g., "FIELDNAME_errors", would be allowed.

To prevent this, I would like to suggest that the "extends=NXobject" tag be removed from the NXroot "definition" tag, i.e., the NXroot class would not inherit anything. This is clearly a backward-compatible change because currently the NXobject class is empty, and I don't believe that it would have any additional side-effects, but I would welcome comments before issuing a PR.