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--)

dune/solvers/iterationsteps/blockgsstep.cc

@@ -44,11 +44,11 @@ void BlockGSStep<MatrixType, DiscFuncType, BitVectorType>::iterate()
     assert(this->ignoreNodes_ != NULL);
 
     if (gs_type_ != Direction::BACKWARD)
-        for (int i=0; i<this->x_->size(); i++)
+        for (std::size_t i=0; i<this->x_->size(); i++)
             iterate_step(i);
 
     if (gs_type_ != Direction::FORWARD)
-        for (int i=this->x_->size()-1; i>=0 && i<this->x_->size(); i--)
+        for (std::size_t i=this->x_->size()-1; i>=0 && i<this->x_->size(); i--)
             iterate_step(i);
 }