Function split_at 

pub fn split_at(
    shape: &Shape,
    at: Point,
    tol: ModelTolerance,
) -> GeoResult<Vec<Shape>>

Split a shape at the point on it closest to at.

Lines, arcs and open polylines yield two shapes; a closed polyline yields one open polyline that starts and ends at the split point. Circles need two points: see split_circle.