Fix #7: Load/Save format for plotting now JSON.

Plotter now stores its layouts/settings in a JSON file (in
.stormtracks), that is easily readable/editable. layout/plot_settings
have been changed from classes to dicts to help this.
N.B. I haven't made this generic, but it would be easy to do. If another
class needs it then it might be worthwile creating a SettingsSaver
class.