Add helper function that makes an empty image

[cdeil]

@keflavich I think it would be nice to have a utility function like make_empty_image to be able to make test images with a single line of code.

Do you agree?
Any thoughts on the API (should cover most of our need to write unit tests)?

[keflavich]

image_utils.wcs_to_empty(shape, wcs=wcs)
and
image_utils.make_empty_image(shape, xcen, ycen, dx, dy, rot) ?