consider adding warning message when using polars

[jgori-ouistiti]

Consider adding a warning message when using polars rather than pandas, possibly in

def get_x_values(data, x) -> set: if isinstance(data, pd.DataFrame): if x is None: return set(data.columns) return set(data[x].unique()) elif data is not None: return set([_ for _ in range(len(data))]) else: return set(x)

Now it exits gracefully, but fails later (in my case in check_order_in_data).