feat: return storage handles from StorageManager

gufe/storage/storagemanager.py

@@ -67,15 +67,21 @@ def append_to_namespace(namespace: str, filename: str) -> str:
         # We opt _not_ to use Paths because these aren't actually path objects
         return f"{namespace}/{filename}"
 
-    def register(self, filename: str):
+    def register(self, filename: str) -> str:
         """Register a filename for later transfer to external storage.
 
         Parameters
         ----------
         filename : str
             The filename to register for transfer.
+
+        Returns
+        -------
+        str
+            The globally namespaced path to be used by other units
         """
         self.registry.add(filename)
+        return self.append_to_namespace(self.namespace, filename)
 
     def load(self, filename: str) -> bytes:
         """Load an item from external storage.

gufe/tests/storage/test_storagemanager.py

@@ -80,7 +80,8 @@ def test_register(self, storage_manager):
         assert filename not in storage_manager
 
         # Register the file
-        storage_manager.register(filename)
+        out = storage_manager.register(filename)
+        assert out == storage_manager.append_to_namespace(storage_manager.namespace, filename)
 
         # Check it's now in the registry
         assert filename in storage_manager.registry