Support for ISM extinction parameters in FitModel

[tomasstolker]

• Support for ISM extinction in FitModel (only with run_multinest). The V band extinction and reddening are free parameters (ism_ext and ism_red) in the bounds dictionary.
• Added plot_size_distributions in plot_mcmc for plotting random samples of the posterior of the grain size distributions from FitModel.
• Added plot_extinction in plot_mcmc for plotting random samples of the posterior of the extinction. Supported by both extinction implementations (i.e. ISM and size distribution).
• The ln_prob values (i.e. posterior value of each sample) are added to the SamplesBox as ln_prob attribute.
• Added apply_ism_ext function to ReadModel.
• Added log_normal_distribution to dust_util.
• Added ism_extinction to dust_util.