Function candidates 

pub fn candidates(
    geometry: &Geometry,
    needs: Needs,
    fill: Fill,
    aspect: f64,
    blocked: &[Rect],
) -> Vec<Rect>

Every rectangle of whole tiles of geometry that holds needs at fill, overlaps none of blocked, is no longer than aspect times its width in sites, and is least: no rectangle a tile row or column smaller inside it holds the needs too. In order of their top row, then left column, then size.