#603 - drawImage() keywords for setting output size.

This PR introduces a few convenience keywords to drawImage() to set the output image size, either by resizing an existing image or by overriding the `getGoodImageSize` values in the case where a new image gets constructed.

Here's an excerpt from the updated docstring:

```
        @param nx           If provided, use to set the x-direction size of the image.  Must be
                            accompanied by `ny`.
        @param ny           If provided, use to set the y-direction size of the image.  Must be
                            accompanied by `nx`.
        @param bounds       If provided, use to set the bounds of the image.
```

There are also a few new unit tests to go along with these.
> Or do you think the scale example above should also be disallowed?

Maybe I'm being inconsistent, but it seems ok to me to allow the scale of the input image to be changed.  The main difference between scale and bounds is that when you change the bounds, any data already in the input image is destroyed.  OTOH, the scale or wcs is kind of an add-on piece of information, so changing it doesn't clobber any previous data.  Plus, not all images even *can* have their bounds changed.  If it is a subImage of a larger Image, then that is an error.  But any image can have its scale/wcs changed without affecting any other object.

Also, there is a historical use case at play as well.  It used to be that you couldn't set the scale of an Image in the constructor.  And in fact, the initial scale used to be 1 if I recall correctly.  So it was common to make the image with some size, and then draw with the scale you want, which would set the scale to that.  Like the example you quoted above, but with no `scale=0.2` in the `ImageD` construction.

Neither of these points seem terribly critical to me, so I'm open to reconsidering this design decision.  Do you think it would be better design to disallow changing the scale in the `drawImage` method **if** the input image is provided and its wcs is not None?