Log message:
Import dune-istl-2.2.0 as wip/dune-istl.

This is the iterative solver template library which provides generic sparse
matrix/vector classes and a variety of solvers based on these classes. A special
feature is the use of templates to exploit the recursive block structure of
finite element matrices at compile time. Available solvers include Krylov
methods, (block-) incomplete decompositions and aggregation-based
algebraic multigrid