message: BugFix: base virt. function signature differed from derived one
diff: class AliVertexGenerator: public TObject {
  public:
  virtual TVector3 GetVertex() = 0;
  - virtual Float_t GetLastVertexTime() { return 0; }
  + virtual Float_t GetLastVertexTime() const = 0;
  ClassDef(AliVertexGenerator, 1) // Base class for vertex generators
  };
  