Forbid negative Hessian values

[hcho3]

Currently, each tree updater tests if each data point has negative Hessian value and ignores the data point with negative Hessian. This complicates the updater code. We should have extra validation steps so as to ensure that all Hessian values are non-negative. The updater code then can be simplified by removing the Hessian check.

[trivialfis]

@hcho3 Is it really doable? Wouldn't the regularization term generate negative value?

[hcho3]

Would regularization cause data points to be ignored?

[trivialfis]

@hcho3 Good point.