Better figure handling for out of order function calls

Quick fix to ensure just_plot() works even if previous steps haven't been properly instantiated (for instance I had to add a function to just_plot that ensures a fig and ax exist, and creates them if not, and now I renamed "create_figure" to "manage_figure" which has an input "use_existing_figure=True" which by default uses the existing figure if this is called out of order and sets things like the figure width, height and dpi without creating a new figure, though it will if use_existing_figure is False.