refactor(Training): remove non-expanding window option (#244)

tests/test_walk_forward.py

@@ -42,9 +42,8 @@ class IncrementingStubModel(Model, BaseEstimator, ClassifierMixin):
         self.window_length = window_length
 
     def fit(self, X, y):
-        assert len(X) == self.window_length
         for i in range(len(X)):
-            assert X[i][0] + 1 == y[i]
+            assert X[i][-1] + 1 == y[i]
 
     def predict(self, X):
         return np.array([row[0] + 1 for row in X])
@@ -66,7 +65,6 @@ def test_walk_forward_train_test():
         X=X,
         y=y,
         forward_returns=y,
-        expanding_window=False,
         window_size=window_length,
         retrain_every=retrain_every,
         from_index=None,