colord: Do not always normalize cd_it8_utils_calculate_xyz_from_cmf()

Also provide functionality to make normalizing easier.
colord: Add a trivial optimisation
colord: Do not interpolate the spectrum out of range

Just return zero for this case.
colord: Provide cd_spectrum_multiply() to multiply two spectra
colord: Set an ID when using cd_spectrum_planckian_new()

This makes it easier to keep track of what it actually is.
colord: Add CdColorUVW and some useful functionality

This is supposed to represent the CIE 1964 color space.