Function project_linf6 

fn project_linf6(
    qxx: &mut f32,
    qxy: &mut f32,
    qxz: &mut f32,
    qyy: &mut f32,
    qyz: &mut f32,
    qzz: &mut f32,
    threshold: f32,
)

L-infinity projection for 6-component symmetric tensor