Adds noise to a single time point in intensity domain
add_noise_to_intensities(spectrum, standard_deviations)a single spectrum from the list obtained using
make_experimental_design.
vector of standard deviations for the random noise
data.table
if the intensity_deviations were provided, then noise is
sampled from a normal distribution with mean 0 and standard deviation
equal tothose deviations and added to Intensity.