When group classes that are valid in any base class are added to the NXobject base class, it is important to exclude NXroot to enforce the rule that only NXentry groups are permitted.

This is an anomalous entry that is the only example in the entire repository where the mechanism for defining classes is conflated with the contents of that class. Its presence here implies that the attribute is required, whereas it is completely redundant.

The NXroot base class is unique in not applying to an HDF5 group, but to the root level of the NeXus file itself.

This also adds fields to the permitted non-NeXus objects at the root level.