New issue 1198: Add ability to plot cell edges

Matthew Turk:

Right now we can plot grid patch edges with `annotate_grids`, but not the cell edges.

This should be straightforward to implement with a flag in the call to the pixelizer. Much like how the line pixelization occurs in the volume renderer, when a pixel is near to the edge of a cell (comparing `pdx` `pdy` `px` and `py`) closer than some epsilon (user specifiable, probably defaulting to something sane and thin) it should toss in a black line. Since the pixelization right now works as returning data values, this would be a separate call that would return RGBA values where the color is either (0.0, 0.0, 0.0, 1.0) or (0.0, 0.0, 0.0, 0.0).

Responsible: MatthewTurk