ENH: Use volume 2 in pdf segmenter

Add the 2nd resampled volume as an additional input of the PDF segmenter.
Factorize setOutputNode in Resample Widget

This allows a cleaner code and the Resample step to work with isotropic
images as input (the step is automatically validated).
Rename vessel extraction to vessel enhancement

-- WIP --

Also revisit this placeholder and update it further now that we know that
it will be used with EnhanceUsingNJetDiscriminantAnalysis.