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- The older versions are not supported anymore.
- apycot is not used anymore.

When one add some non-composite relation to be followed, it should be
automatically added to cw_skip_copy_for else it will be both deep copied and
swallow copied, as demonstrated by the amended test.

We have to consider it at every levels of the container tree, not only for the
top-level entity.

While doing so, it sounded good to move creation of the list to a cached
property to avoid redoing this for each cloned entity of the tree.

cw_skip_copy_for on an entity class allow to skip non final relations in
Entity.copy_relations(). Extend this behavior to final relations in compound
clone, this allow to skip (i.e. not set) some attributes on the cloned
entities.

This can occur in real cases when etype is related to his parent by multiple
optional relations where only one could be filled. We could generate 'OR'
expressions but for now this is not handled.

The compound graph may be used to generate RQL expressions that can be used in
permissions. E.g. when permissions depend on permissions on container.

Use a shortest path algorithm to generate RQL expressions from any etype to the
container of the graph using exclusively mandatory relations.
Make expressions more readable by using variables based on etype name.