hierarchy analysis

* Files are grouped in documentation folder (in PPP-NLP-classical)
* __Hierarchy_analysis.md__: first proposition of prepocessing of the tree to limit the number of possible dependencies. The current Hierarchy analysis has been tested on about 50 examples (+ the examples of the manual)
* __Algorithm.md__: proposition of overall algorithm
* __Review.md__: my first comments on the relevance of Hierarchy_analysis (on other things...).