Implement spectral component of reflectance model

[sammorrell]

Currently, the reflectance model in the MCRT model is colour-insensitive. To make the models as physically accurate as possible, we need to add in the spectral reflectance of each of the materials. We can do this by loading in the reflectance curves from a file and sampling the PDF to determine the ratio of photon packets to kill off at a given wavelength.