added function to plot a random set of samples from chains
updated plotting; titles on all plots, and output all to a single pdf
save chains to a pkl file for any further diagnostics after the fit
function to plot models corresponding to quantiles

takes the 16th and 84th quantiles of each parameter, then plots
models corresponding to all combinations of those values.
Also plots the model corresponding to the 50th quantile of
all parameters, plus the data