Support saving a static image in Jupyter widget for interactive-figure creation

[pkgw]

When creating an interactive figure for the AAS journals, the interactive should be accompanied by a static image, which stands in for the interactive experience in static formats (PDF) and in cases where the user is unable to experience the interactive (slow internet, accessibility limitations). pywwt should make it easy to create such a static view in the same way as the aas-timeseries tool.

The Qt widget has the render method, but I don't believe there is an equivalent in the Jupyter version (possibly in part due to #210?). We expect interactive-figure creation to be done primarily through the Jupyter interface, so this functionality should be added. If the Jupyter call signature can match that of the Qt version, that would be desirable. (More desirable, I think, than presenting the same API as aas-timeseries.)