Models referenced by a SED-ML document should be retrieved and flattened, if needed and possible, upon instantiation #517
Description
When creating a SED-ML document from a CellML or a SED-ML file, we need to make sure that all the CellML models referenced by a SED-ML document are available. Then, for each referenced CellML model, we need to make sure that they can be flattened, if they have imports.
This doesn't apply to our JavaScript bindings.