Function to_nurbs_within 

pub fn to_nurbs_within(
    model: &mut Model,
    shape: &Shape,
    tolerance: f64,
    tol: Tolerances,
) -> OgeomResult<Built>

Rebuild a solid with every surface and curve in B-spline form: exactly where a closed form exists, as to_nurbs does, and fitted within tolerance where none does (an offset surface, a helix, an offset curve), where to_nurbs refuses.

A fitted curve is same-parameter with the curve it replaces; a fitted surface has its own parameterization, and every trim on it is re-derived.

Errors

OgeomError::Construction if tolerance is not a distance; OgeomError::NotDone if a fit cannot reach it; as to_nurbs.