add Neo4j graph storage adapter

[YuliaVyaznikova]

No description provided.

[AsphodelRem]

1. Insertion into the database currently fails when nodes contain cluster information. Neo4j reports an error because only primitive property types are supported.

2. Could we use a field from the entity class as a Neo4j label? It would be useful to filter entities by their type. One possible approach is to add a get_label() method to the base Node class that returns the field to be used as the Neo4j label.

[YuliaVyaznikova]

Now clusters are serialized to JSON for Neo4j compatibility, and get_label() on Entity returns self.entity_type as a Neo4j label (e.g. :PERSON).