Storage module: expose Node objects (#634).

DeepaMehtaObject API:
{{{
Object getDatabaseVendorObject();
}}}
Returned is the Neo4j `Node` object that underlies the DeepaMehtaObject.

Required by the `dm4-geospatial` module.

IMPORTANT: stop the DM server before rebuilding the dm4-storage-neo4j module.

See #634.