Configuration for the system
Example config file:
# use diskplan_config::ConfigFile;
# let config_text = r#"
# "#;
# let config: ConfigFile = config_text.try_into().unwrap();
# let stem = config.stems.get("main").expect("no main stem");
# assert_eq!(stem.root().path().as_str(), "/tmp/diskplan-root");
# assert_eq!(stem.schema().as_str(), "simple-schema.diskplan");