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).