enhance rewrite_shared_optional so one can specify where the new identity relation should be added (used by cw multi-sources planner)

introduce make_constant_restriction function, useful to build a restriction without adding it yet to the tree

it wasn't doing what it should have done (see cw.rset related_entity
implementation)

We actually can't use distinct if an attribute is selected  and we sort
on another attribute

      which may cause pb later (at sql generation time for instance)