Replace `sierpinski_foam` with `sierpinski_foam2`, and drop the 2

[jgostick]

sierpinski_foam2 has a more sensible api where you actually specify the image size you want, and it crops accordingly. I think it would be smart to remove the original one and replace it with v2. The docstring of the v2 function could explain how to get a proper full image that is not cropped. There is also an issue about how to do the cropping in the v2 function using a mode (#895).