add figure saving to image-utilities

[TimMonko]

Using the screenshot (for 3D) and figure_export (for 2D), allow saving of pngs (and more, with imageio) of the images in the viewer. This is intended to have 'reproducibility' for image saving and creating figures. This would allow consistent contrast, window zoom, and 3D viewer positioning, along with image resolution.

I hope to have this feature available soon, but my goals make for a larger project than I have the ability to prioritize, right now.