[graph] Exact match in search
When searching for documents from the graph, the search should match the exact form of label and its subforms. Currently, in case of Ngrams with N>1, parts of the string can also be matched and display irrelevant documents. For exemple, "parents d’enfants scolarisés" will match docs with "parents" or "enfants". Test on CNRS instance 0.0.5.8