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.
Added step-54 to list.
Fixed html typo in step-52
Merge pull request #227 from luca-heltai/tutorial-list-fix

Added step-54 to tutorials page.
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
Merge pull request #228 from kronbichler/master

Adjust Intel compiler results for new vectorized accumulation
Fix a few minor textual issues.
Give a brief outline of what the program will actually do.
Reformat and rewrite parts of the documentation to make it fit more our
currently preferred style (in particular, of documenting whole code blocks as
opposed to individual lines).
Adjust some text.
Adjust image size to ensure that pictures fit into a typical window.
Provide links to step-54.
Also build on step-53.
Build the OpenCASCADE wrapper documentation even if the library wasn't
configured with this external dependenct.
Break overlong lines.
Merge pull request #230 from bangerth/updates-to-step-54

Updates to step 54
Added clear() to TableHandler
Merge pull request #231 from alrashedf/master

Added clear() to TableHandler