[Pallav-Biyala]

Fixes #3296

Summary

• Changed the default value of the sample parameter in TimeSeries.values() from 0 to None to distinguish between callers who omit the argument and those who explicitly select a sample.
• When sample is omitted for a stochastic time series, emit a warning explaining that only sample 0 will be returned and suggesting TimeSeries.mean() or TimeSeries.quantile(0.5) when a point forecast is desired.
• Preserve backward-compatible behavior: calling values() still returns sample 0 by default. Explicitly specifying sample=0 or another sample does not trigger the warning.
• Updated the method's docstring to document the new default and warning behavior.
• Added four tests covering the warning and returned values for stochastic time series, explicitly specified samples, and deterministic time series.
• No changes were needed to TimeSeries.to_dataframe(), which already represents samples of a stochastic time series in separate columns.

Testing

• darts/tests/test_timeseries.py: 329 passed, 1 skipped.
• git diff --check: passed.

[dennisbader]

Thanks for the fix and your first contribution @Pallav-Biyala 🚀

I slightly updated the warning message and tests. Apart from that everything looks great 💯

I'll merge once all tests have passed