Grouping by property ID should be applied to:
- Statements
- Qualifiers
- Reference Snaks
Perhaps grouping should always be applied; in that case, the ungrouped structure should still be accepted as input.
Note: The grouped structure is better for access by property. The an ungrouped list map of statements is better for access by GUID.
https://github.com/wmde/WikibaseDataModelSerialization/issues/50