Default values of certain site_symmetry items #23
Description
Throughout the dictionaries, site symmetry data items are generally assigned the 1_555 value as the reasonable default. During the migration from DDL1 to DDLm, this default value for the following items was removed:
_restr_plane_site_symmetry
_restr_plane_class_displacement_max_site_symmetry
_restr_rigid_body_site_symmetry
Furthermore, site symmetry items now usually import the {'file':templ_attr.cif 'save':site_symmetry} attribute set, although this might be deviated from if unique _description.text attribute values need to be retained. In that case, it would be recommended to at least copy the rest of the attributes from this attribute set for consistency.