Some improvements/changes in the one-class SVM solver:

Some improvements/changes in the one-class SVM solver:

- use quickselect and quicksort instead of heap to shorten the running
   time on finding violating pairs. See details in a new section in
   supplementary materials of our linear one-class SVM paper.

- modify the comparison function so instances with the same gradient
   values are ordered by their instance indices. This ensures the same
   results by using different implementations (e.g.,
   quickselect/quicksort vs. heap)