Fix dropping columns with zero var in transform

[Asiaticum]

I have added one line in transform()

#### This is where you need to drop columns that have zero variance ######
self.cols_zero_variance = X_sel.columns[(X_sel.var()==0)]
if len(self.cols_zero_variance) > 0:
        print('    Dropping %d columns due to zero variance...' %len(self.cols_zero_variance))
        X_sel = X_sel.drop(self.cols_zero_variance, axis=1)
+        df = df.drop(self.cols_zero_variance, axis=1)

In the subsequent steps, feature selection using Sulov and XGBoost is performed on the df.
If df.drop(self.cols_zero_variance, axis=1) is not executed, there is a possibility that features not present in X_sel will be selected, which could lead to a KeyError when return X_sel[self.features], y_sel is executed.