Function rgsl::fit::wmul

pub fn wmul(
    x: &[f64], 
    xstride: usize, 
    w: &[f64], 
    wstride: usize, 
    y: &[f64], 
    ystride: usize, 
    n: usize
) -> (Value, f64, f64, f64)

Returns (Value, c1, cov11, sumsq).