History length control added to PlotGroupPanel. The parameter maximum_plot_history_length now controls what is saved in history list as follows:
        If the parameter maximum_plot_history_length is -1, history aving s unlimited
        (except by your machine memory!!). If it is 0, then no history is saved. If
        it is N, then up to N plots are saved in history list.
Merge pull request #505 from mjabri/master

Plot history length control; defaults to keeping 20 plots instead of all plots, to avoid filling up memory.