Revert deprecation of GeometryType::BasicType for 2.6

See merge request !88

(cherry picked from commit 1a0afaff)

Merge branch 'feature/require-cmake-3.1' into 'master'

See merge request !83

[cmake] Require CMake 3.1 similar to dune-common

See merge request !82

(cherry picked from commit 20d93270)

0a683268 [cmake] Require CMake 3.1 similar to dune-common

[Changelog] Mention arbitrary refinements.

See merge request !78

added CHANGELOG.md for release

See merge request !76

Merge branch 'bugfix/topologyfactory' into 'master'

See merge request !75

[bugfix] `TopologySingletonFactory` not uses `std::unqiue_ptr` internally

See merge request !74

(cherry picked from commit b2e3318a)

4b8e78c0 [bugfix] `TopologySingletonFactory` not uses `std::unqiue_ptr` internally

Merge branch 'feature/overhaul-topologyfactory' into 'master'

See merge request !73

overhaul `TopologySingltonFactory`

See merge request !72

(cherry picked from commit 54e9de10)

cda80d4c overhaul `TopologySingltonFactory`

.gitlab-ci.yml: build against releases/2.6 branch of other modules

See merge request !70

[bugfix] Avoid always falling into slow code branch in AxisAlignedCubeGeometry

See merge request !68

The methods `corners` and `global` of AxisAlignedCubeGeometry
suffered from wrongly implemented switches that caused the
implementation to always use the slow code branch for cubes
embedded into a higher dimensional world space. This is only
a performance bug as that branch will still compute the correct
result.

[ci] Stop testing with Debian 8

See merge request !65

[cleanup] fix  extra ‘;’ warning

See merge request !67

This fixes warning:  extra ‘;’ with -Wpedantic in GCC.

Remove GeometryType::BasicType

See merge request !66

[geometrytype] Deprecate GeometryType::BasicType

See merge request !64

[bugfix] Restore reference semantics of ReferenceElement methods

See merge request !61

Make *GeometryTypeIndex constexpr

See merge request !63

Make all methods constexpr. For some, this requires (at
least for older compilers) to replace 'if' by a ternary
operator.

With the new constexpr GeometryType, we probably don't need the BasicType
anymore. This patch deprecates it.

Improve the description of BasicType::none

See merge request !58

[cleanup][skip ci] Add indentation info

See merge request !62

The deprecation tag in doxygen is here to warn users. We cannot
mark the method as deprecated directly, because only the return
type will change.

By using decltype(auto) we forward references of the
implementation as references. This reverts the interface
change introduced in !52 in case the implementation returns
references. As a consequence the all tests in dune-localfunctions
pass again.

For (possible) implementations returning values these will
also be forwarded. In the future (after a grace period of
one release), this should be switched to references completely
as proposed in !52.

replace uses of GeometryType::make* by non-deprecated implementations

See merge request !60