fix(WalkForward): major bug where we passed in "window of windows of data" is resolved, refactored walk_forward_train_test() and load_data()

utils/typing.py

@@ -0,0 +1,7 @@
+from typing import Protocol
+
+class SKLearnModel(Protocol):
+    def fit(self, X, y, sample_weight=None): ...
+    def predict(self, X): ...
+    def score(self, X, y, sample_weight=None): ...
+    def set_params(self, **params): ...