add changelogs from release to master

See merge request core/dune-istl!148

use `std::make_unique`

See merge request core/dune-istl!147

[test] Add check for BCRSMatrix::nonzeroes()

See merge request core/dune-istl!93

added empty changelog

See merge request core/dune-istl!145

[solvers.hh] Always convert the reduction to double explicitly.

See merge request core/dune-istl!138

This already happened for most solvers when setting the reduction in the
result struct.  It is may be needed when the field type is some extended
number type that prohibits implicit conversion to `double`.  This commit
brings the remaining two solvers in line that did not do this yet.

Fix the #include mess in solvers.hh

See merge request core/dune-istl!137

[preconditioners] fix type of CGSolver reduction parameter after recent updates

See merge request core/dune-istl!132

[bugfix] ensure nonzeroes always return the number of nonzero entries

Closes #28

See merge request core/dune-istl!127

[Warnings] Fix MPI 1 deprecation warnings (by switching to MPI 2).

See merge request core/dune-istl!125

fix ambiguity of _1 for certain compilers

See merge request core/dune-istl!128

[preconditioner] fix ILDL for SIMD types

See merge request core/dune-istl!129

the weight is a scalar, but the current code defines it a field_type,
which might be a SIMD type or something else, which does not behave as
a scalar. We fix this by using the SimdScalar traits to extract the
scalar type and use this.

[Warnings] Fix a signed/unsigned warning.

See merge request core/dune-istl!126

This fixes warnings of the following form about functions deprecated in MPI 2.
```
/home/joe/Projekte/dune-simd/dune-istl/dune/istl/paamg/test/parallelamgtest.cc: In function ‘int main(int, char**)’:
/home/joe/Projekte/dune-simd/dune-istl/dune/istl/paamg/test/parallelamgtest.cc:217:3: warning: ‘int MPI_Errhandler_create(void (*)(ompi_communicator_t**, int*, ...), ompi_errhandler_t**)’ is deprecated: MPI_Errhandler_create is superseded by MPI_Comm_create_errhandler in MPI-2.0 [-Wdeprecated-declarations]
   MPI_Errhandler_create(MPI_err_handler, &handler);
   ^~~~~~~~~~~~~~~~~~~~~
In file included from /home/joe/Projekte/dune-simd/dune-common/dune/common/parallel/mpitraits.hh:23:0,
                 from /home/joe/Projekte/dune-simd/dune-common/dune/common/parallel/plocalindex.hh:9,
                 from /home/joe/Projekte/dune-simd/dune-istl/dune/istl/paamg/test/anisotropic.hh:8,
                 from /home/joe/Projekte/dune-simd/dune-istl/dune/istl/paamg/test/parallelamgtest.cc:7:
/usr/lib/x86_64-linux-gnu/openmpi/include/mpi.h:1345:20: note: declared here
 OMPI_DECLSPEC  int MPI_Errhandler_create(MPI_Handler_function *function,
                    ^~~~~~~~~~~~~~~~~~~~~
```

bump version to 2.7-git [ci skip]

See merge request core/dune-istl!123

Comment out warnings

See merge request core/dune-istl!121

These warnings are very irritating. They do not
warn about deprecated or misused feature. They are
issued any time you compile the tests. They don't
provide any useful information unless you look at the
code and have insight in the implementation detains.

To sum up: They don't look like a warning addressed to
the user, but like a reminder on an intended future
implementation addressed to the implementor. Hence
they should not be exposed to the user.

If anyone feels that this is an important user warning
feel free to re-enable it after adding enough information
making the warning actually useful.

[bugfix] Avoid calling std::real for unsupported types

See merge request core/dune-istl!120

The condition estimate is only supported for field_types
float and double. Unfortunately, compiling this failed
for any field_type not supported by std::real. Replacing the
dynamic check by a static one helps to avoid the problem.

[ci] Stop testing with Debian 8

See merge request core/dune-istl!118

Feature/cg condition estimate

See merge request core/dune-istl!102

[bugfix] silence signed/unsigned comparison warning

See merge request !119

[bugfix] Make sure to use tolower from c

See merge request !116

[cleanup] Use range for instead of std::for_each

See merge request !117

This commit replaces all use cases of std::for_each(..., std::mem_fn(...))
by a much more readble plain range for. It does not replace other uses,
where this may be more controversial.

[bugfix] correct Givens rotations after SIMD vectorization

See merge request !114