Implement plotting methods for the model object

[benRenard]

For instance:

• Obs vs. sim at some or all gauges, e.g. model.plot_obs_vs_sim(gauge=['ID1', 'ID2'])
• simulated fields (either q or states) at specific time steps, e.g. model.plot_sim(field='q',time_step=["1998-05-23", "1998-05-24", "1998-05-25"])
• same for inputs, e.g. model.plot_input(field='prcp',time_step=["1998-05-23", "1998-05-24", "1998-05-25"])
• mesh properties, e.g. model.plot_mesh(attribute='flwdir')
• parameter fields, e.g. model.plot_par(par=['cp','ct'])
• and surely many others...

[nghi-truyen]

Yes, or we can even create a sub-package smash.plot to do this..