Tests for 18.5.1. Get Relationship by ID and 18.5.2. Create relationship.
Test for 18.5.3. Create a relationship with properties.
Tests for 18.5.4. Delete relationship
18.5.5. Get all properties on a relationship
Tests for 18.5.6. Set all properties on a relationship
Test for 18.5.7. Get single property on a relationship
Test for 18.5.8. Set single property on a relationship
Rename GetProperty() to Property()
Test for 18.5.9. Get all relationships
Test for 18.5.10. Get incoming relationships
Test for 18.5.11. Get outgoing relationships
Test for 18.5.12. Get typed relationships
Test for 18.5.13. Get relationships on a node without relationships
Remove big Relationship test, replaced by many small tests in relationship_test.go
Completed test for 18.6.1. Get relationship types
Remove deprecated test that was replaced by last commit.
Test for 18.7.1. Set property on node
Split entity interface and baseEntity struct.
Split NodeIndex into generic index and NodeIndex.
Add RelationshipIndex support.  No tests yet.