Rename fields in Relationship model

[buske]

In this issue, @julesjacobsen suggested renaming the fields in Relationship to be:

• subject: identifier of the subject Individual; equivalent to the Biolink "Subject"
• relative: identifier of the relative Individual; equivalent to the Biolink "Object"
• relationship: the relationship the subject has to the relative (e.g., if the subject is the relative's parent, then relationship would be a descendant of isBiologicalParent[Of])

[buske]

Now that the terms in the KIN ontology are directional, updating these property names is helpful for clarifying this direction. I went ahead and committed this change, but realized now that this breaks the FHIR IG, so think we should hold off on this change until discussing at/after Connect.