#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.
>if the user provides either bounds or nx,ny, it probably doesn't make much sense to let scale be automatically calculated as the Nyquist scale.

This is reasonable.  I added a check to drawImage to require `scale` or `wcs` when providing `nx`, `ny` or `bounds`.