he id value, these 3 next values 
  are used in Vicon Workstation and Nexus to distinguish events' type.  The value 0 is for
  general event. Value 1 corresponds to the foot strike event and value 2 for the foot off.

  This function can also returns an updated list of events.
  EVENTS = BTKAPPENDEVENT(H, LABEL, TIME, CONTEXT)
  EVENTS = BTKAPPENDEVENT(H, LABEL, TIME, CONTEXT, SUBJECT)
  EVENTS = BTKAPPENDEVENT(H, LABEL, TIME, CONTEXT, SUBJECT, DESCRIPTION)
  EVENTS = BTKAPPENDEVENT(H, LABEL, TIME, CONTEXT,