Fixed case where function didn't return where it should

I added these return statements so that invalid inputs or valid end cases would no longer continue running through the rest of the function.

Author: nbrgr

machine_learning/decision_tree.py

@@ -48,12 +48,15 @@ def train(self, X, y):
             return
         if y.ndim != 1:
             print("Error: Data set labels must be one dimensional")
+            return
 
         if len(X) < 2 * self.min_leaf_size:
             self.prediction = np.mean(y)
+            return
 
         if self.depth == 1:
             self.prediction = np.mean(y)
+            return
 
         best_split = 0
         min_error = self.mean_squared_error(X,np.mean(y)) * 2