#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.
Also, I hadn't thought about this before, but 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.  That's probably going to result in unexpected results for them.  I think we should require scale or wcs to be given if they provide parameters that indicate the desired image size.  After all, what does that size mean without some kind of scale?  Probably the Nyquist scale is not what they would expect.