Create scaler tree correctly.

[Lunderberg]

Currently, scaler events are thrown into the event tree (GEB) or ignored entirely (EVT). These should make it into a scaler tree instead.

• TDetector should have a virtual function IsScaler(), which defaults to false. Any class that should be added to the scaler tree should override this function to return true.
• TWriteLoop should check the return of IsScaler() when creating branches and filling, to perform each action on the appropriate tree.