Introduce Individual_Id

[stschiff]

We had a productive discussion yesterday with @nevrome and @TCLamnidis about using a new Individual_Id field to indicate whether samples/datasets belong to the same individual. We currently can encode this via the Relation_* field. Introducing an additional Individual_Id would make these cases more explicit and add a name to individuals.

Things to consider:

• Our validator should check within packages that samples belong to identical individuals need to be consistently marked via both the Relation_* fields and the new Individual_Id.
• We would not currently indicate in the schema any constraints across packages, as the schema strictly only defines single packages and not collections or archives. The PCA and PMA, in contrast would have to come up with additional policies how to use the new field.
• Would it be desirable to make even cross-archive links possible via the Relation_* and Individual_Id possible? Certainly not in the schema, but worth considering.
• The validator needs to make sure that forged packages, which may introduce additional links, are valid.