Hall A Analyzer

To help with maintaining binary compatibility for future patch releases of a given main release, add a pointer to an extension data structure to THaAnalysisObject. By default, this is a null pointer. If a bug fix or new feature in a certain class requires new data members (something which happens rather often), its constructor would allocate a structure containing those data members. They would be accessed like

MyDetectorExtraData* ex = static_cast<MyDetectorExtraData*>(fExtra);
if( ex->fSomeData > 0 )
  // do something

The fExtra pointer should be to some kind of "extra data" base class that offers a minimal common API, e.g. to find out the stucture's size, stream the info etc. Maybe it is easiest to let the base class just be TObject.