MERGE

[TIDY] move ApiNgrams class from views/api/analytics.py to a new file: views/api/ngrams.py because it corresponds to the generic route /api/ngrams and not only for analytics usage

[FEAT] new view/route for adding an ngram via PUT 'api/ngrams/?text=mynewngramstring' or adding and indexing via PUT 'api/ngrams/?text=mynewngramstring&corpus=42'

[GRAPH EXPLORER] Better types to check parameters and give user some usefull informations. I extensively use the Either type of Haskell that is needed to easyly understand the logic of the process.