node and edge: support for belonging to more than one rz-doc

[alon]

requirements

1. each node can belong to one or more rz-docs
2. each edge can belong to one or more rz-docs

   corollaries

3. an edge whose endpoints belong to rz-doc a' doesn't necessarily belong to rz-doc a'

   non implementation

a. relationship

use a 'belongs to' relationship between neo-node(rz-doc) and neo-node(rz-doc node) why: breaks corollary 1

implementation possibilities

1. attribute: add 'rz-doc' list attribute on each rz-doc_node and rz-doc_relationship
2. relationship (non-implementation a) plus attribute on relationship
3. relationship plus have each edge represented twice, once as relationship and once as neo-node