v0.9.3: allow a root_hint to suggest_root_model

SLayer 0.9.3

You can now pass a root_hint to force recommend_root_model to use a model you have already picked as the query root. If the hint can reach every requested item it is honored outright; if it cannot, SLayer falls back to its automatic pick and warns you which items the hint could not reach. A hint that names a model that does not exist, is the wrong kind, or belongs to another datasource is rejected with a clear error.