add RGB and RGB arithmetics
in RGB, separate interface and definition
make all rel_equal constexpr
use pass-by-value instead of pass-by-reference
Spectrum: separate interface from implementation
change return type of Spectrum::at()/[]() to plain float, because there are infinite wavelengths within each bin
inline calls to detail::bin_min_wavelength
rid Interval::min()/max(), instead publish min_/max_ as const public members
extract the interpolation stuff into a separate class