Implement std::tuple_element for MultiTypeBlockVector and Matrix

See merge request !459

Both behave like std::tuple, and currently even inherit from std::tuple.
According to

  https://en.cppreference.com/w/cpp/utility/tuple_element

we are allowed to specify std::tuple_element for our own types.

Add `operator*` for `ScaledIdentityMatrix` and scalar

See merge request !457

Export the cholmod_factor object

See merge request !455

This is needed for some of the more advanced features of CHOLMOD.

refactor the way python is used in dune

See merge request !432

comment out CMP0087 NEW cmake policy because it fails on some CI runners

correctly set CMP0087 cmake policy to NEW for cmake >= 3.14

fix ci

pep 42: use implicit namespace packages which make it possible to combine subpackages using editable (source) and non editable (pypi)

only use a cmake install a python module in the SKBUILD setting - for an install from build dir pip will take take of this since it is part of the setup.py file

add a stopgap solution unti ci repo up and running again

Update requirements.

add dune packages to requires in setup.py.in

[testing]

only add dune modules that have been found as requirements in setup.py.in

removed setup.py.in since the default is provided in dune-common

adapt ci yml file to python bindings on by default

disable python bindings for clang 5 (ubuntu 18) since no numpy matching numpy version can be installed from the gitlab registry

[bug fix python bindings]
fix an issue with 'entry' being used in implicit build mode but ready==build

[minres] Compute preconditioned residual also for initial residual

See merge request !454

Bump version to 2.9-git

See merge request !451

Make sure that the relaxation factor for smoothers never is complex.

See merge request !447

Remove signed-unsigned comparison on repartition.hh

See merge request !448

to get rid of a compiler warning.

The note

/dune-istl/dune/istl/paamg/smoother.hh:130:26: note: the ABI of
passing structure with complex float member has changed in GCC 4.4

for DefaultConstructionArgs::getArgs got me thinking. I just could not
imagine why there should be comlex value involved. Turned out we
somehow assumed that for complex the relaxation factore would be
complex, too. IMHO a relaxation factor should scale and hence be real.
This MR make ssure that the relaxation factor is never complex.

Unforntunately, then a note for std::make_pair with a
complex in repartition.hh appeared:
/usr/include/c++/8/bits/stl_pair.h:524:5: note: the ABI of passing
structure with complex float member has changed in GCC 4.4
     make_pair(_T1&& __x, _T2&& __y)

and I just resorted to constructing Pair directly. Not sure whether
this is really needed but it should not hurt and ensures more silence.

Get rid of shadowed/unused variable/typedef gcc-8 warnings

See merge request !445

Get rid of unused/shadowed variable compiler warnings.

See merge request !442

[gitignore] Ignore macOS caching files

See merge request !439

[MultiTypeBlockMatrix] Export the constructor of std::tuple

See merge request !395

[changelog] Mention SuiteSparse potentiel slowdown with openBlas

Closes #91

See merge request !437

Fixes #91

Update list of authors

See merge request !438

Cholmod: Add support for almost all matrix types

See merge request !424

This is achieved by using the flat matrix and vector forEach implementations.
The unit test is extended by a multi type example.

The routines traverse the container recursively and call a functor
at each scalar entry passing also the flat index.

Remove some warnings about unused typedefs and suggested braces

See merge request !435