GENE — Instances of OntologyEntries need a way to be further qualified

Some ontologies, such as GeneOntology, don't have this concept but others,
such as MGED's, do.

Request from a number of sources to modify the model:

• add a new class named "Slot" to the Description package with the
  following:

• attribute: "name" of type String, required
• parent base class: Extendable
• association to Measurement named "Measurement"
  Slot end:
• aggregates Measurement end
• cardinality: 1..1
  Measurement End:
• name: "measurement"
• cardinality: 0..1
• navigable
• association to OntologyEntry named "SlotValue"
  Slot end:
• aggregates OntologyEntry end
• cardinality: 1..1
  OntologyEntry end:
• name: slotValue
• cardinality: 0..1
• navigable
• add rule that Slot has either measurement or ontologyEntry, exclusive

• OntologyEntry has association to Slot named "Slots"
  OntologyEntry end:

• aggregates Slot end
• cardinality: 1..1
  Slot end:
• name: slots
• cardinality: 0..N
• navigable

An simpler alternative would be an association from OntologyEntry to
itself named "Associations":
1st OntologyEntry end:

• aggregates 2nd OntologyEntry end
• cardinality: 1..1
  2nd OntologyEntry end:
• name: associations
• cardinality: 0..N
• navigable

(MAGE ISSUE 7)

see below