Return model path to `filename`, checking locally first then in the config file.

def modelpath4file(filename, ext:str='.tgz'):
    "Return model path to `filename`, checking locally first then in the config file."
    local_path = URLs.LOCAL_PATH/'models'/filename
    if local_path.exists() or local_path.with_suffix(ext).exists(): return local_path
    else: return Config.model_path()/filename