libcolord: Add cd_spectrum_get_value_raw()
libcolord: Add cd_spectrum_normalize_max()

Usually you just want to normalize the spectrum to a maximum certain value,
rather than at a specific wavelength.
libcolord: Add cd_spectrum_get_resolution()

This returns the resolution (the frequency or time interval) of the spectrum.
libcolord: When using cd_it8_add_spectrum() replace any with the same ID
libcolord: Allow a SPECTRAL_START_NM value of zero
libcolord: Only set SPECTRAL_NORM when the normalisation is enabled
libcolord: Use double precision for SPECTRAL_START_NM and SPECTRAL_END_NM