Function py_blatt_weisskopf 

pub fn py_blatt_weisskopf(
    tags: &Bound<'_, PyTuple>,
    decay: &PyDecay,
    l: &Bound<'_, PyAny>,
    reference_mass: f64,
    q_r: f64,
    sheet: &str,
    kind: &str,
) -> PyResult<PyExpression>

Construct a Blatt-Weisskopf amplitude.