Fix headercheck by adding missing include.
Remove more unused typedefs.

They caused warnings with g++ 4.8.
[LFSIndexCache] Replace C arrays that might be of length 0 with array
class

The standard says that C-style arrays of length 0 are not supported,
which triggers a warning in LFSIndexCache if the underlying LFS has
no children.

The std::array container does support zero-length arrays, however, so
this patch just replaces the C array with a Dune::array (which is either
a std::array or a std::tr1::array on all platforms supported by PDELab
and should thus have the correct specialization for zero length).

Thanks to Christoph Grüninger for the tip-off.
[lfsindexcache]
add typedefs for ContainerIndex and DOFIndex
[backend/istl]
add new dg-amg backend

Kudos to Peter for *_amg_dg_backend.hh
[backend/istl]
dg-amg: fix the construction of the conforming-FEM matrix
Merge branch 'feature/dg-amg'