Stricter interpretation of the "UniquenessConstraint", refs 1463

[mwjames]

This PR is made in reference to: #1463

This PR addresses or contains:

• Checks that no competing values on the same subject/property are declared for when the property is specified as [[Has uniqueness constraint::true]]

This PR includes:

• Tests (unit/integration)
• CI build passed

[mwjames]

For example denoting [[NDL ID::00629192]] and [[NDL ID::00629193]] on the same subject will no longer be permitted. The first assigned value is attributed to be "valid" (given that no other subject has made a uniqueness claim for this property value) and any other competing value to the same property is declared as invalid.

https://sandbox.semantic-mediawiki.org/wiki/Issue/2113

[kghbln]

Now documented.