Allow for optional asym_id in tables where information can be the same across different instances of the same entity

[brindakv]

For tables like ihm_starting_model_details, the same starting model can be used for different instances of the same entity.

Currently, the schema requires the information to be explicitly provided for all instances, which can result in repetition.

Such cases can be simplified by allowing for asym_id to be optional and a new optional data item can be added to indicate whether the property applies for all instances of the same entity.