Documentation bump
Camera added

Implemented as a transformer of Scenes, though right now just on
ListScenes.  Additional generality could be useful.
Added assignment-specific camera
Initial camera rays can be generated

...given focal length, width and height of focal plane (in world
units), and width and height of image (in pixels)
Swapped x and y in rays function

I think that would have resulted in a transposed image, since the
final list would have been (0, 0), (0, 1), (0, 2), etc. instead
of (0, 0), (1, 0), (2, 0), etc.
Also added exports for RasterProp and rays
Pipline complete

PPMs can now be written, we're mapping the trace across the view rays.
Unfortunately, the produced image is all background.  Debug time.
Object and ListScene made Show instances

The camera transform looks like its working on the scene ok, but
it's hard to tell with just numbers.
Success! A little...

Rays were being generated in the opposite direction.  Fixed by
putting the focal plane at -f z instead of +f.  This got me more
than just blue (good!), but seemingly the wrong area of the image
(bad!).  On the upside, the image seems to be proportional.