function index2FiraName(firaval) {
  const weightIndex = Math.floor(firaval/3);
  const widthIndex = firaval%3;
  const firaWeights = [ 'Two', 'Four', 'Eight', 'Hair', 'Thin', 'UltraLight',
  'ExtraLight', 'Light', 'Book', 'Regular', 'Medium', 'SemiBold', 'Bold',
  'ExtraBold', 'Heavy' ];
  const firaWidths = [ 'Normal', 'Condensed', 'Compressed' ];
  return [ firaWidths[widthIndex], firaWeights[weightIndex] ];
}
Convert the parameter index number to Fira width and weight
@param { number } firaval
@returns { [ string, string ] }