class THaEventHeader

 THaEvent

 This is the base class for "event" objects, which define the
 structure of the event data to be written to the output DST
 via ROOT object I/O.

 Any derived class must implement at least the following:

  - Define all member variables that are to appear in the output
    for each event (a la ntuple).
  - Set up the fDataMap array to link these member variables to
    global analyzer variables.

 Data members can be simple types (Double_t etc.) and any ROOT objects
 capable of object I/O (i.e. with a valid Streamer() class).
 For any data members other than simple types defined in fDataMap,
 the Fill() method has to be extended accordingly.