Function fit_voxel 

fn fit_voxel(
    te: &[f64],
    y: &[(f64, f64)],
    logy: &[(f64, f64)],
    b0: f64,
    params: &DecomposeParams,
    lm: &mut LmScratch,
) -> (f64, f64, f64, f64, f64, f64)

The three-stage alternating fit for a single voxel. Returns (C+, C−, C₀, R2*₀, χ+, χ−).