_item.mandatory_code is inconsistent for three data names

[jamesrhester]

_item.mandatory_code for _array_structure_list_section.array_id is implicit in the definition for _array_structure.id. but mandatory in the definition block for _array_structure_list_section.array_id. mandatory is not a possible value for _item.mandatory_code so I assume it should be given as implicit in both places?

Likewise for _diffrn_scan_frame_monitor.frame_id : implicit in the parent definition but yes in the individual definition.

Likewise for _variant.variant_of: implicit in the parent definition but no in the individual definition.