namespace.

This namespace was probably meant to mark implementation details.  However it
led to violations of the ODR (one definition rule), and to warnings about
defined-but-unused functions (see below).

To illustrate the problem consider the function template
Dune::readMatrixMarket:

  template<typename T, typename A, int brows, int bcols>
  void readMatrixMarket(Dune::BCRSMatrix<Dune::FieldMatrix<T,brows,bcols>,A>& matrix,
                        std::istream& istr)
  {
    // ...
    readSparseEntries(matrix, istr, entries, header, NumericWrapper<T>());
  }

The ODR states (§3.2/5):

  There can be more than one definition of a [...] non-static function
  template (14.5.6) [...] in a program provided that each definition appears
  in a different translation unit, and provided the definitions satisfy the
  following requirements.  Given such an entity named D defined in more than
  one translation unit, then

  - each definition of D shall consist of the same sequence of tokens; and

  - in each definition of D, corresponding names, looked up according to 3.4,
    shall refer to an entity defined within the definition of D, or shall
    refer to the same entity, after overload resolution (13.3) and after
    matching of partial template specialization (14.8.3), except that a name
    can refer to a const object with internal or no linkage if the object has
    the same literal type in all definitions of D, and the object is
    initialized with a constant expression (5.19), and the value (but not the
    address) of the object is used, and the object has the same value in all
    definitions of D; and

  [...]

This is violated by e.g. both readSparseEntries, which is defined in the
anonymous namespace.  Since it is not defined within readMatrixMarket, it must
refer to the same entity in all translation units.  However, its name is
effectively Dune::<unique>::readSparseEntries, where <unique> is unique to
each translation unit, and thus it effectively refers to a different entity in
each translation unit, and the program is ill-formed as soon as move than one
translation unit does #include <dune/istl/matrixmarket.hh>.

The warnings by g++-4.9 looked like this:
======================================================================
g++ -std=c++11 -DHAVE_CONFIG_H -I. -I../../..  -pthread -I/home/joe/Projekte/pdelab-2.4/dune-common -I/home/joe/Projekte/pdelab-2.4/dune-common -I../../.. -I/usr/lib/openmpi/include -I/usr/lib/openmpi/include/openmpi -pthread -DMPIPP_H -DENABLE_MPI=1   -g -O3 -Wall -MT matrixmarkettest-matrixmarkettest.o -MD -MP -MF .deps/matrixmarkettest-matrixmarkettest.Tpo -c -o matrixmarkettest-matrixmarkettest.o `test -f 'matrixmarkettest.cc' || echo './'`matrixmarkettest.cc
[...]
In file included from matrixmarkettest.cc:11:0:
../../../dune/istl/matrixmarket.hh:501:10: warning: ‘void Dune::{anonymous}::readNextLine(std::istream&, std::ostringstream&, Dune::{anonymous}::LineType&)’ defined but not used [-Wunused-function]
     void readNextLine(std::istream& file, std::ostringstream&, LineType& type)
          ^
In file included from matrixmarkettest.cc:11:0:
../../../dune/istl/matrixmarket.hh:601:19: warning: ‘std::istream& Dune::{anonymous}::operator>>(std::istream&, Dune::{anonymous}::NumericWrapper<Dune::{anonymous}::PatternDummy>&)’ defined but not used [-Wunused-function]
     std::istream& operator>>(std::istream& is, NumericWrapper<PatternDummy>& num)
                   ^
======================================================================

(cherry picked from commit 1a19f1c1

)
Signed-off-by: Carsten Gräser <graeser@dune-project.org>