Add serialization for statistic.
Add serialization for statistic.

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