UML Class Diagrams for EiEvent and Associated Classes, Operation Payloads (WD15, ln. 643-649)

This issue is retained as a container for the listed subtasks. All comments prior to the subtask splits are here; the subtasks have independent resolutions.

a. Add mechanism for VEN to query the VTN to find what signals to expect for a specific program (e.g, pg 76 of OpenADR 1.0 has name of the type from EventInfoType).
b. Better describe the relationship between Fig 9 and Fig 10.
c. Describe the difference between EISendEventPayload and EISentEventPayload
d. Allow mechanism for interactions between VEN and VTN for transactions/ For example, the eventStateID in OpenADR 1.0 is intended to handle transactions by avoiding data payload issues. Other option (may be better) is to create a framework that allow creation of multiple XSDs for each of the types.
e. Add mechanism to handle Schema versions? (schemaVersion in OpenADR 1.0)
f. Add mechanism to identify test event (testEvent in OpenADR 1.0)