Update spectral_data.h to use C++17 inline constexpr arrays

[TzuChieh]

Doing this should make the arrays' address irrelevant, so the compiler can potentially do more optimizations.

[TzuChieh]

We now have new color space implementations and decide to keep the original design, as it cleanly hides how the spectral data is loaded. This should not be a performance critical part.