neo4j: add `with_labels` to load node having multiple labels in one Cypher query.

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
neo4j: add `save_entity` shortcut in NeoBatch.

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
src: Introduces `neo` a module for save and load Nit models intro a Neo4j graph database.

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
tests: introduces `test_prog` a program that build a fake documented model to test model related tools.

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
tests: add `test_neo`

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>