REV should adapt to aspect ratio of image

[jgostick]

Some images are thin, like of fibrous media. In these cases the REV function will use cubes with a maximum size equal to the thin dimension. It might be handy to include a mode argument that controls this a bit. For instance, mode='equal' means cubes as we do now, mode='maximal' would mean cubes when smaller than min(im.shape), but uses cuboids when needed, and lastly mode='proportional' would mean the cube always has the same aspect ratio as the image. There would probably be other modes we could add in future....maybe spheres?

[jgostick]

BTW, this relates to a discussion we had recently: #578