Function field_mapping_single_echo 

fn field_mapping_single_echo(
    phase: &[f64],
    mag: &[f64],
    mask: &[u8],
    metadata: &ScanMetadata,
    config: &FieldMappingConfig,
    _n_voxels: usize,
    nx: usize,
    ny: usize,
    nz: usize,
    vsx: f64,
    vsy: f64,
    vsz: f64,
    progress: &mut dyn FnMut(usize, usize),
) -> Result<FieldMappingResult, PipelineError>

Path C: Single echo unwrap