Fix loading of attribute-model association from files.
These 2 changes provide defense in depth against associations read from files when no model manager is specified: 1. First, the `ResourceSetLoader` now has a constructor that allows you to specify a model manager. 2. Second, if `EntityRef` objects with a null model manager are created at load time and *then* a reference model manager is set on the attribute system, use that model manager when returning values.
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