Surface calculation is not unit safe

[brittonsmith]

When calculating a surface, there is no check that the surface value is in the same units as the surface field. The script below shows this:
http://paste.yt-project.org/show/7152/

This will give a surface of size zero when the units are not the same and a non-zero sized surface when they are the same. I'm listing this as a bug because it's simple to fix.