Add second template parameter to FE_Nothing.

For historical (and probably not very good) reasons, finite element classes
have a second template argument denoting the space dimension. FE_Nothing never
got this argument and so it wasn't possible to use it in codimension-one
situations. This patch adds the second argument to make it possible to use
it in these situations.

There is not much that one can test this element with, other than ensure that it
compiles. The next commit will actually use the element in a concrete situation.
Augment documentation.
Use an FE_Nothing instead of FE_DGQ(0).

We don't actually want to use any kind of finite element here at all,
but need to because FEValues requires us to when all we want are the
JxW values. FE_Nothing is cheaper to construct than FE_DGQ(0), so
use it in GridTools::volume().
Merge pull request #223 from bangerth/add-second-template-arg-to-fenothing

Add second template arg to fenothing