Optimize small things in density estimation trees
Optimize small things in density estimation trees

While doing other work, I noticed a few things that could use
improvement:
  * An std::vector that is used as an array
  * Unnecessary dynamic memory allocation
I fixed these things and measured a marginal speedup.

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