fix vector models folder

For a first time user the "vector" directory within the "models" folder does not exist yet, which throws an error when the script tries to iterate over it to list all the vector models downloaded (if any).  This simply creates the directory if it doesn't yet exist.

src/gui_tabs_models.py

@@ -36,6 +36,8 @@ def __init__(self, parent=None):
             models_dir.mkdir(parents=True)
 
         vector_models_dir = models_dir / "Vector"
+        if not vector_models_dir.exists():
+            vector_models_dir.mkdir(parents=True)
 
         existing_vector_directories = {d.name for d in vector_models_dir.iterdir() if d.is_dir()}