CART optimized

- CART code optimized
[Current performance benchmark](https://gist.github.com/mazumdarparijat/1caf9bee2ec1dbf9e07b)
I am a little bit concerned about the values in `simple_labels` not being initialised. Also, since there is an if in the loop below, it is not straightforward all the elements in `simple_labels` will be initialised.

@mazumdarparijat, will all the elements in `simple_labels` be initialised after the loop below? If they are not, is it guaranteed somehow that the non-initialised ones will never be accessed below?