add multi-freq example

examples/benchmarks/LightGBM/features_sample.py

@@ -0,0 +1,9 @@
+import datetime
+import pandas as pd
+
+
+def resample_feature(df: pd.DataFrame) -> pd.DataFrame:
+    df = df.droplevel(level="instrument")
+    df = df.loc[df.index.time == datetime.time(13, 1)]
+    df.index = df.index.normalize()
+    return df