Function spread_model 

fn spread_model<R: Scalar>(
    index: &SwapIndex,
    minus: &SwapIndex,
    fixing: NaiveDate,
    payment: NaiveDate,
    mean_reversion: f64,
    correlation: CmsSpreadCorrelation,
    strike: f64,
    view: &PricingView<'_, R>,
) -> Result<SpreadModel<R>, PricingError>