[DenseVector] Fix compilation error in axpy() implementation
[DenseMatrix] Make sure to always initialize the row and column sizes
[DenseBackend] Make data structures compatible with basic ISTL solvers

Right now, the dense backend cannot really be tested because there are
no solvers. Fortunately, ISTL places only a very small amount of
requirements on the containers within its solvers. So this patch makes
it possible to use ISTL solvers (with a Richardson preconditioner) by
adding the following three features:

- Export a field_type from VectorContainer
- Implement MatrixContainer::mv()
- Implement MatrixContainer::usmv()
[testdensebackend] Solve system using ISTL

With the newly added ISTL solver support in the DenseBackend,
it is now possible to apply a simple ISTL solver with a Richardson
preconditioner to the DenseBackend data structures, so let's do
that to make sure we have actually assembled something reasonable.