Defitinition of `_geom_angle.atom_site_label_2` should import `atom_site_id`

[vaitkus]

All *.atom_site_label_* data items in the coreCIF dictionary (except for _geom_angle.atom_site_label_2) import the common set of attributes from the atom_site_id save frame of the templ_attr.cif template file.

I guess that _geom_angle.atom_site_label_2 does not do this to retain a unique description (_description.text attribute) which explicitly identifies it as the vertex atom in the angle. However, I think that the unique description could be retained even with the import statement in place if all other overlapping properties are removed and the import statement is instructed to ignore duplicate attributes. Either way, the _geom_angle.atom_site_label_2 is also identified as such in the GEOM_ANGLE category example.