[MINOR] Performance tomeklink builtin function (task-parallel)
[MINOR] Performance tomeklink builtin function (task-parallel)

This patch makes a minor performance improvement at script level for the
tomeklink builtin function for under-sampling. In detail, we now avoid
unnecessary sorting and use task-parallel for loops. Later, we might
switch the manhatten distance with the euclidean dist() in order to
fully vectorize the distance computation and exploit matrix
multiplications.

On a machine with 112 vcores, and a scenario of calling tomeklink on a
10K x 1K matrix, the runtime improved from 342s to 45s.

apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml