Use std::size_t instead of int

This avoids the following compiler warnings:

| .../dune-solvers/dune/solvers/iterationsteps/blockgsstep.cc:47:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
|          for (int i=0; i<this->x_->size(); i++)
| .../dune-solvers/dune/solvers/iterationsteps/blockgsstep.cc:51:49: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
|          for (int i=this->x_->size()-1; i>=0 && i<this->x_->size(); i--)