Function model_clean 

fn model_clean<R: Scalar, C: YieldCurve<R>>(
    bond: &FixedRateBond,
    curve: &C,
    reference: NaiveDate,
) -> Result<R, CurveError>

The clean price bond takes on curve, per 100 face, in mode R, where it settles from reference: the flows after settlement carried there.