Modify Guido's change in r22657 so that we only really provide the

Modify Guido's change in r22657 so that we only really provide the
additional functions for TriaAccessor<0,1,spacedim>, not for the more
general class TriaAccessor<0,dim,spacedim>. This moves us in the
direction where we ask 1d cells for cell->face() which will return an
object of the kind TriaAccessor<0,1,spacedim>.

We could in principle implement the more general case as well, but
that would only be useful if we want to do get the end points of a
face in 2d, for example (TriaAccessor<0,2,spacedim>) but there is no
use for such a thing in dimension independent programming.

git-svn-id: https://svn.dealii.org/trunk@22689 0785d39b-7218-0410-832d-ea1e28bc413d