self.train_y0 not found when setting model to LinReg

[Mustardburger]

When setting the model option in the json config file to LinReg, the program throws an error of self.train_y0 not being defined. This is because self.train_y0 is only defined when the model is set to CellBox, as the build function in CellBox instantiates self.train_y0. Otherwise, in other models, self.train_y0 is never instantiated. This also applies to self.monitor_y0 and self.eval_y0.