offsetHSL method
Implementation
Color offsetHSL(double h, double s, double l) {
getHSL(_hslA);
_hslA["h"] = _hslA["h"]! + h;
_hslA["s"] = _hslA["s"]! + s;
_hslA["l"] = _hslA["l"]! + l;
setHSL(_hslA["h"]!, _hslA["s"]!, _hslA["l"]!);
return this;
}