21.2 electron mva selector

@cbecot @strichte @jmaurer @jpoveda @psommer @christos Adding AsgElectronSelectorTool to support DNNs for electron ID using lwtnn. The AsgElectronSelectorTool (adapted from AsgElectronLikelihoodTool) reads the configuration file and retrieves the variables. The TElectronSelectorTool registers all cuts and applies them. The DNN score is calculated in the ElectronDNNTool. It inherits from the ElectronSelectorTool which can later be used to support other MVA methods like CNNs or DNNs using ONNX instead of lwtnn.