Add uncertainty in spectra to the emission structure within the model

[sammorrell]

It's becoming apparent that we should be trying to account for the variability in light sources with the MCRT modelling. So, a simple way to do this would be to incorporate optional bounds into the spectra that we feed into the emission. Then for each light source, if the bounds are present, we can sample a single spectrum that lies between the bounds instead of just using a single for each light source.