Name: event-definition
Type: request
Purpose: >
  Defines ("creates") a new event.
Response: event-definition-confirmation
Inherits: message
Elements:
  Signature: { required: true }
  From: { required: true }
  Group-Id: { type: identifier }
  Event-Data: { required: true, type: object, class: event }
Rejection-Codes:
  21: Incomplete event data
  22: Invalid event date
  23: Invalid value for estimated turnout
  31: Sender is not authorized to define event here
  41: Group does not exists