[Backend] Add dense backend with vector and matrix wrappers

This patch adds a very simple backend that provides scalar dense matrices
and vectors. Both wrappers are templated on their storage engine, which
has to be a std::vector-like container in that it provides similar constructors,
element access via [], a size() method and standard iterators. The matrix
is currently in row-major format.

Right now, you need a recent C++11-compatible compiler with support for template
aliases and std::bind to use this backend.
[Tests] Add testdensebackend for new dense backend

The new test is adapted from testpoisson, but doesn't perform a solve
for lack of a solver.
[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.
[Backend] Extract uncached vector views to an external class
[DenseBackend] Switch VectorContainer to common UncachedVectorViews
[ISTL] Switch ISTLBlockVectorContainer to common UncachedVectorViews
[Backend] Extract uncached matrix views to an external class

Like the vector views, the matrix views are not very container-specific
as long as they don't attempt to perform any clever caching and just
forward most operations to the global container, so this patch extracts
them into external utility classes. Along the way, we also fix a bug
where some modifier methods were erroneously declared const and also
finally provide a const view for matrices that was missing until now.

Just as for the vector views, the new components deprecate global_container()
in favor of container().
[DenseBackend] Switch MatrixContainer to common UncachedMatrixViews
[ISTL] Switch ISTLMatrixContainer to common UncachedMatrixViews

If possible, we just provide a template alias to the utility component.
If the compiler does not support template aliases, the fallback code
defines nested classes that inherit from the utility classes.
Merge branch 'feature/add-dense-backend'

This branch mainly brings in a new, simple backend for dense
vectors / matrices that use std::vector-like objects for storage.
While writing that backend, I noticed that the local views for the
vectors and matrices are mostly backend-agnostic as long as they
don't perform any buffering, so I turned those into a utility component
that is now shared between ISTL and the new backend. Finally, I grabbed
the opportunity and added const views to matrices, something that was
missing until now (and it was basically free in terms of work...).

* feature/add-dense-backend:
  [ISTL] Switch ISTLMatrixContainer to common UncachedMatrixViews
  [DenseBackend] Switch MatrixContainer to common UncachedMatrixViews
  [Backend] Extract uncached matrix views to an external class
  [ISTL] Switch ISTLBlockVectorContainer to common UncachedVectorViews
  [DenseBackend] Switch VectorContainer to common UncachedVectorViews
  [Backend] Extract uncached vector views to an external class
  [testdensebackend] Solve system using ISTL
  [DenseBackend] Make data structures compatible with basic ISTL solvers
  [DenseMatrix] Make sure to always initialize the row and column sizes
  [DenseVector] Fix compilation error in axpy() implementation
  [Tests] Add testdensebackend for new dense backend
  [Backend] Add dense backend with vector and matrix wrappers