Inconsistency between HNode and HLink as models of roassal objects

[ClotildeToullec]

In a HV, a Roassal shape (RSBox for example) has a HNode as its model. It is thus easy to access it.
A RSLine however, has an association as model. The association key and value are HNodes. The HLink is not easily accessible.

A remark from Soufyane: 2 HNodes can have several HLinks between them, represented by only 1 RSLine. Should the model of aRSLine be a collection? This problem should be adressed in order to fix this issue.