merge branch lowrank back into trunk

doc/LOWRANKBRANCH-BACKPORT-CHANGES

dune/solvers/common/arithmetic.hh

dune/solvers/common/genericvectortools.hh

dune/solvers/iterationsteps/multigridstep.hh

dune/solvers/iterationsteps/truncatedblockgsstep.hh

dune/solvers/norms/diagnorm.hh

dune/solvers/norms/energynorm.hh

dune/solvers/norms/fullnorm.hh

dune/solvers/operators/lowrankoperator.hh

dune/solvers/operators/nulloperator.hh

dune/solvers/operators/sumoperator.hh

dune/solvers/solvers/loopsolver.cc

dune/solvers/test/lowrankoperatortest.cc

dune/solvers/test/sumoperatortest.cc

.../solvers/transferoperators/compressedmultigridtransfer.hh

dune/solvers/transferoperators/genericmultigridtransfer.hh

...olvers/transferoperators/truncatedcompressedmgtransfer.cc

dune/solvers/transferoperators/truncatedmgtransfer.hh