plot all ensemble members as histograms

historical_simulation and future_scenarios now have option to provide a density_plot = True or False. By default, this is set to True for historical_simulation, and adds density information in the form of a histogram to figure 3 at the point of realizations being ranked (usually today's date). For future_scenarios, it is set to False, and adds histogram of concentration to scenarios 1-3, histogram of leaching and concentration to scenario 4-5, and rug plot of ages to scenario 6.