Allow Centroid() with sparse vectors.
Allow Centroid() with sparse vectors.

ersanliqiao/mlpack,lezorich/mlpack,stereomatchingkiss/mlpack,thirdwing/mlpack,Azizou/mlpack,thirdwing/mlpack,darcyliu/mlpack,Azizou/mlpack,stereomatchingkiss/mlpack,darcyliu/mlpack,Azizou/mlpack,thirdwing/mlpack,chenmoshushi/mlpack,minhpqn/mlpack,ajjl/mlpack,trungda/mlpack,darcyliu/mlpack,BookChan/mlpack,minhpqn/mlpack,BookChan/mlpack,theranger/mlpack,trungda/mlpack,bmswgnp/mlpack,erubboli/mlpack,lezorich/mlpack,chenmoshushi/mlpack,ajjl/mlpack,theranger/mlpack,ranjan1990/mlpack,erubboli/mlpack,ranjan1990/mlpack,ajjl/mlpack,trungda/mlpack,BookChan/mlpack,minhpqn/mlpack,bmswgnp/mlpack,palashahuja/mlpack,datachand/mlpack,stereomatchingkiss/mlpack,palashahuja/mlpack,chenmoshushi/mlpack,ersanliqiao/mlpack,datachand/mlpack,ersanliqiao/mlpack,palashahuja/mlpack,datachand/mlpack,bmswgnp/mlpack,ranjan1990/mlpack,lezorich/mlpack,erubboli/mlpack,theranger/mlpack