Modifications to Provenance feature branch

Based on the discussion in #35, I did the following:
1. Removed ParentCitationID from Citations and its relationship
2. Created RelatedCitations entity for consistency with how we are
storing other relationships
3. Added VersionCode to RelatedResults
4. Removed VersionCode and VersionOfDatasetID from DataSets entity
5. Created RelatedDatasets entity with a VersionCode for consistency