#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.
>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?

I'm leaning that direction, but not strongly.