tauRecTools: remove member variables in TauPi0ScoreCalculator
To calculate the BDT score, we need to provide a list of input variables. Previously, the input variables are declared as memeber variables, which make the class stateful. These member variables are now replaced with local variables.
Add a new class, i.e. BDTHelper, to help calculate the BDT score. This class is similar with TRTBDT in HelperFunction. In the future, we will remove TRTBDT.