Constant TUKEY_SOURCE 

const TUKEY_SOURCE: &str = "χ-separation toolbox v1.1.3 Chisep_script.m:69-73 \
    (\"tukey_fac = 0.4; % Recommendation: Siemens, GE: 0.4, Philips: 0\"); QSM.rs utils::tukey_window docs";