Colourmixing multiple sources RGBAW

See https://github.com/norbertrostaing/BlinderKitten/issues/19

And Declan Randall's lectures on LED Metamers  https://blog.etcconnect.com/why-do-metamers-matter + https://www.youtube.com/watch?v=nwcEKCEwR8k&t=2s