Update pyretailscience/segmentation.py

Co-authored-by: codiumai-pr-agent-pro[bot] <151058649+codiumai-pr-agent-pro[bot]@users.noreply.github.com>

Author: mvanwyk

pyretailscience/segmentation.py

@@ -145,6 +145,8 @@ def __init__(self, df: pd.DataFrame, segment_col: str = "segment_id") -> None:
 
         """
         required_cols = ["customer_id", "total_price", "transaction_id", segment_col]
+        if "quantity" in df.columns:
+            required_cols.append("quantity")
         contract = CustomContract(
             df,
             basic_expectations=build_expected_columns(columns=required_cols),