Add more idiophones

[jonikorpi]

• Celesta: basically like a piano, but without stretched overtones or sympathetic strings
  • apparently has only 2nd and 6th harmonics: it’s a metallic idiophone, so I guess the overtones match a glockenspiel?
• Gong
  • https://www.researchgate.net/figure/Spectrum-of-bonang-gong-Sunda-main-spectral-components-no-1-16_tbl1_221494065
• Gamelan metallophone
  • https://physics.byu.edu/docs/publication/639