cutout and save the current Aladin view as a FITs image with WCS

[havok2063]

It would be cool to be able to save a cutout of a specified size of the current view in Aladin as a FITS image that contains the WCS information. One could then overlay this image on any other image data.

or alternatively, using the get_JPEG_thumbnail method that currently exists, to save the JPEG image but embed the WCS info into the image header, so the JPEG itself can be overplayed elsewhere.

[tboch]

We will add a get_view_WCS() (and a get_dimensions()) method to retrieve an Astropy WCS object for the current view