Nice object to manipulate StanfordCoreNLP results

* Define the object. 
* Define a function converting the output of the Stanford library into this object. 
* Add a pretty printer (allowing the direct use of `print` fucntion).
You do a lot of step in this function. Why not explode it into a lot of small private function that will be more easy to read and use meaningful function names that would allow to avoid the need for comments?