kernel_weighted_sum_np now supports a version of leave-one-out sums even if num_train != num_eval

kernel_weighted_sum_np supports being called with weighted_sum = NULL, in which case it will avoid computing the actual sums :)

ls ccdf cv now uses a (possibly slower) block-based algorithm to avoid excessive memory consumption.
added a 'memfac' argument to npcdistbw, which controls the memory consumption used by the algorithm. Tweaking it may lead to performance enhancements.