Add optional lower-bound clipping for ResidualDouble quantiles

[kallal79]

Adding opt-in response_lb clipping in ResidualDouble so quantile/interval outputs respect non-negative support (for example response_lb=0.0), with regression tests added and passing.

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[fkiraly]

Good feature!

But: This clips only the quantile predictions, but not the probability predictions.

It would be better if you modify the output of _predict_proba appropriately - to a truncated distribution.