Add support for plotting in particular axes in `BasicCurve`

[miguelgondu]

It would be great to have the affordance of plotting a curve in the same axis as we e.g. plot volume. Inspired by seaborn, I will implement something like

_, ax = plt.subplots(1, 1)
geodesic, _ = m.connecting_geodesic(p0, p1)
geodesic.plot(ax=ax)