Spectral and XYZ Color Functions

This is the beginning of a set of functions for working with color as wavelengths (spectral color) and in the xyY and XYZ color spaces.

Current functionality includes:

spectrumLabel - Add a color bar containing the visible spectrum to a plot's X axis.

spectrumRGB - Converts wavelengths of light to equivalent RGB values (in the sRGB space).

colorMatchFcn - Raw data of popular color matching functions (spectra to XYZ) for various standard observers:
* CIE 1931 2-degree
* CIE 1964 10-degree
* CIE 1931 2-degree modified by Judd (1951)
* CIE 1931 2-degree modified by Judd (1951) and Vos (1978)
* Stiles and Burch 2-degree (1955)
* Stiles and Burch 10-degree (1959)

illuminant - Raw data for the A, D65, and E standard illuminants.