Introduce a class that can't be used.

Introduce a class that can't be used.

Specifically, declare DoFInvalidAccessor that is the accessor
for cases where dim>spacedim -- i.e., things that can't logically
happen, but that at times do happen in code like

  for (quad=0; quad<GeometryInfo<dim>::quads_per_cell; ++quad)
    cell->quad(quad)->do_something();

Here, the code is not executed in 1d, but we want it to be syntactically
correct so that we can compile the function. Having a DoFInvalidAccessor
then serves the same purpose that InvalidAccessor already does for
tria accessor functions: such objects cannot be created but rather
throw an exception, but they provide the correct syntax.