Split spectral input formats into spectrum only and spectrum+alpha. Issue #344.

There are two types of input formats for spectral (color) inputs: InputFormatSpectralReflectance and InputFormatSpectralIlluminance.

The goal is to split these two input formats as follow:
InputFormatSpectralReflectance: the input evaluator will only emit a spectrum
InputFormatSpectralIlluminance: the input evaluator will only emit a spectrum
InputFormatSpectralReflectanceWithAlpha: the input evaluator will emit a (spectrum, alpha) pair
InputFormatSpectralIlluminanceWithAlpha: the input evaluator will emit a (spectrum, alpha) pair