set default filename for plot_residuals like other plots

[mdaeron]

I need to go back to the various plotting functions, but you're right that they lack consistency. The thing to decide first is what should be the default behavior?

• return a matplotlib Figure object by defaut, which means that most aspects of the figure may be modified before displaying/saving it; return None and save to disk with some optional parameter (e.g. output = 'savefig')
• return None and save to disk by default; return a Figure object with some optional parameter (e.g. output = 'fig')

In both cases, output = 'ax' could be used to ask for a matplotlib Axes object, which allows including the plot in a larger figure.

Any opinion on the best default behavior? Any change needs to be deliberate because it will (slightly) break backward compatibility.

[japhir]

I made an issue to discuss what to do, then this can remain the "implementation" branch, which might end up trashed ;-).