FIX objective missing a constant term when D>16

[michaelchughes]

This does NOT affect the quality of the cluster centers and/or sample assignments. Just the computed value of the objective function.

In pairwise_distance()" function in KMeansRexCore.cpp. When D > 16 you do not add the norm of the data points to the distance matrice. The fix is to insert the following line as the last line of the "else" case:
Dist.colwise() += X.square().rowwise().sum().col(0);

Reported by Sobhan ([email protected])