Fix debug build to address issue #10

Travis fails to build on gcc due to wrapping.  Creating PR in the meantime since issue is unrelated to this fix.
There are lots of good reasons to treat Ground as a body whenever possible. OSRF did it the other way and is now planning to switch because their code has so many special cases in it that could have been avoided. Here I don't think the real exceptional case is Ground, it should also include any bodies which are welded to Ground by a Weld mobilizer (that is, any bodies that have nq=nu=0).