Adding numpy count_nonzero & minor template fix

Have i missed something?
This comment is either to much or not enough in my opinion, I mean I had to think hard about what it means here. As I understanding it, it is to say that adding a bool to an int is ok. I don't feel a need for a comment for that, but if you think it is worthwhile, then the comment should be "adding a boolean to an int is defined in the standard".