Add an interactive GUI

[nobodyinperson]

A GUI to control a NumericalModel interactively would help a lot debugging and testing. Especially desired features are being able to:

• change the model variables, parameters and forcing on the fly (Gtk.Scale)
• directly see the model output (matplotlib.pyplot)
• control simulation speed (Gtk.Scale)
• pause and continue or do stepwise simulation (Gtk.Button or Gtk.ToggleButton)