Function py_flatte 

pub fn py_flatte(
    tags: &Bound<'_, PyTuple>,
    mass: PyParameter,
    observed_channel_coupling: PyParameter,
    alternate_channel_coupling: PyParameter,
    observed_channel_daughter_masses: (PyMass, PyMass),
    alternate_channel_daughter_masses: (f64, f64),
    resonance_mass: &PyMass,
) -> PyResult<PyExpression>

Construct a Flatte amplitude.