Compute the means and stds of `self.cont_names` columns to normalize them.

def apply_train(self, df:DataFrame):
        "Compute the means and stds of `self.cont_names` columns to normalize them."
        self.means,self.stds = {},{}
        for n in self.cont_names:
            assert is_numeric_dtype(df[n]), (f"""Cannot normalize '{n}' column as it isn't numerical.
                Are you sure it doesn't belong in the categorical set of columns?""")
            self.means[n],self.stds[n] = df[n].mean(),df[n].std()
            df[n] = (df[n]-self.means[n]) / (1e-7 + self.stds[n])