Implement construction of BDMatrix from std::initializer_list

See merge request !108

.gitlab-ci.yml: update environments

See merge request !106

resolve ambiguous call by explicitly calling `Dune::Hybrid::size`

See merge request !107

Reference: core/dune-common#77

- Added:   Debian 10 with gcc in C++17 mode
- Removed: Debian 8 + Backports with clang

Reference: http://lists.dune-project.org/pipermail/dune-devel/2017-July/002224.html

[GeneralizedPCGSolver] Increment iteration counter for all verbosity levels

See merge request !105

This commit fixes Issue #32

[bugfix][cmake] install allocator.hh

See merge request !103

Commit 8da92ce1 added allocator.hh but
forgot to add it to the installed headers in CMakeLists.txt.

[multirhstest] Check with AlignedNumber.

See merge request !87

[Fix] Update mailing lists addresses.

See merge request !100

Add `data()` accessor methods

See merge request !99

Feature/support mingw

See merge request !88

[dynamic polymorphism] use #if instead of #ifdef for the guard

See merge request !95

we add a compatibility layer, when
DUNE_ISTL_SUPPORT_OLD_CATEGORY_INTERFACE is enabled. We now ise #if
instead of #ifdef to check for the flag and add the switch to
scalar-product interface class.

fix the cmake magic to enable backwards compatibility for the category interface

Closes #29

See merge request !94

Use correct type for second template argument of `std::array`

See merge request !90

Using `unsigned long` instead of `size_t` only works on architectures
where the two types are identical, but not on, for example, i386.  On
architectures where the types differ, the generic version would be
used instead of the specialization for `std::array`.

Bug-Debian: https://bugs.debian.org/860666

[bugfix] add missing header

See merge request !89

if the AMG had a non-standard allocator, the template parameters didn't
match, as the hierarchy was assumend to use the default allocator.

this is necessary to ensure that the solver fulfills the alignement
requirements of the vector

basically everywhere the type is hard-coded to double, only in the
hierarchy it is deduced from the MatrixOperator, which lead to alignement
errors when using vector types.

this is necessary to support other data types

This makes sense since we do test with AlignedNumber now.  If Vc is found, we
still test with that, in addition to AlignedNumber.