Allow individual noise or response functions to be plotted on their own
Response functions can
draw their data to an existing
axis provided by a
Solution, but they can't just plot themselves on their own. Furthermore, if you provide the
axis of a figure you made yourself (e.g. with
Solution.noise_figure()), the function doesn't add axis labels or grids etc.
The functions should be able to access the same plotting tools that
Solution uses. Maybe split the plotting stuff out of
Solution and into somewhere else where it can be used by the functions separately.