[FEATURE] Tests to create graphs optimized with SQL (thx Mathieu for your...

[FEATURE] Tests to create graphs optimized with SQL (thx Mathieu for your input): ok. Need to connect to explorer now.

tests/cooc_whitelist.sql

+SELECT
+    COUNT(*) AS cooccurrences,
+    ngX.terms,
+    ngY.terms
+FROM
+    node_node AS n  -- the nodes who are direct children of the corpus
+    
+INNER JOIN
+    node_node_ngram AS nngX ON nngX.node_id = n.id  --  list of ngrams contained in the node
+INNER JOIN
+    node_node_ngram AS whitelistX ON whitelistX.ngram_id = nngX.ngram_id -- list of ngrams contained in the whitelist and in the node
+INNER JOIN
+    node_ngram AS ngX ON ngX.id = whitelistX.ngram_id -- ngrams which are in both
+    
+INNER JOIN
+    node_node_ngram AS nngY ON nngY.node_id = n.id
+INNER JOIN
+    node_node_ngram AS whitelistY ON whitelistY.ngram_id = nngY.ngram_id
+INNER JOIN
+    node_ngram AS ngY ON ngY.id = whitelistY.ngram_id
+    
+WHERE
+    n.parent_id = %s
+AND
+    whitelistX.node_id = %s
+AND
+    whitelistY.node_id = %s
+AND
+    nngX.ngram_id < nngY.ngram_id   --  so we only get distinct pairs of ngrams
+    
+GROUP BY
+    ngX.id,
+    ngX.terms,
+    ngY.id,
+    ngY.terms
+ORDER BY
+    cooccurrences DESC
+LIMIT
+    200
\ No newline at end of file