Start the image viewer.
The image viewer will be started, if found. Calling this function when the viewer has already been started will not cause a second viewer to be started. The image viewer will be started automatically by any of the commands like `image_data` .
Image visualization is optional, and provided by the DS9 application  .
-  http://ds9.si.edu/site/Home.html
See the bugs pages on the Sherpa website for an up-to-date listing of known bugs.
- load_image, pack_image, unpack_image
- image_model, image_model_component, image_source, image_source_component
- contour, contour_data, contour_model, contour_ratio, contour_resid, image_close, image_data, image_deleteframes, image_fit, image_getregion, image_ratio, image_resid, image_setregion, image_xpaget, image_xpaset