Make fastamg work with Dune::SeqILU0<..> and Dune::SeqILUn<..>

These two preconditioners differ from the iterative preconditioners
and are not suitable for fastamg. With the added specialization,
ILU preconditioners may e used with fastamg, but there is no speedup
from the optimizations in fastamg. For compatibility, we still want
these specializations.