Allow for related Key Events to share information

[jabedini@rti.org]

The new data model must explicitly include the concept of related KEs whereby they share some (but not all) of the same features, which could represent a source of crosstalk between AOPs containing those KEs that is distinct from the sharing of a single KE.

This data model should also facilitate the reuse of this shared information among those related KEs to avoid increasing the burden on KE authors.