improve nth element speed by using bucketselect or cutting plane instead of quickselect

this work http://www.math.grin.edu/~blanchaj/Research/ABGS_KSelection.pdf , and a few other works such as http://arxiv.org/pdf/1104.2732.pdf, propose gpu accelerated algorithms for nth element selection.
I believe using bucketselect presented in Alabi et al. would result in a significant speed up while keeping the code simple and elegant.
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