Vocabulary Creation

this part is responsible for creating the vocab to be used for training the classifier 
you dont push back into certain indicies. you just push back into the end of the Mat.
```
Mat a, b;
a.push_back(b); // now a has b appended at the end
a.push_back(b); // now a has two copies of b at the end
```
just make sure you dont give a size to the empty matrix in the begining