Function month_point 

fn month_point<C: InflationCurve<f64> + ?Sized>(
    index: &str,
    month_start: NaiveDate,
    fixings: &FixingsView<'_, f64>,
    curve: &C,
) -> Result<f64, CurveError>

One monthly index point: a published fixing where there is one, the curve otherwise.