changes from my branch

I added code to extract spectra and add them to the props structure.

I also added code to run monte carlo simulations and add the calculated errors to the props structure. I had to fix realize_noise to make the variances of the original and generated noises similar. Otherwise it was scaling the mean, which isn't what you want to do. There's a better way to calculate the errors -- below I've done the things the stupid way, but you could iterate over tags in the structure for more streamlined code.