Revert deprecation of GeometryType::BasicType for 2.6

See merge request !88

As there are still some use cases that cannot be easily switched from using GeometryType::Basictype
to plain GeometryType, revert its deprecation for the 2.6 release.

This does **not** revert the deprecation of the GeometryType constructor from BasicType.

Enforce use of correct floating point type in AxisAlignedCubeGeometry

See merge request !85

used dune macros to suppress deprecated warnings

See merge request !84

YaspGrid can use non-double floating point type, but AxisAlignedCubeGeometry
sneaked in some doubles...

[cmake] Require CMake 3.1 similar to dune-common

See merge request !82

[bugfix][test] return 1 on failure

See merge request !81

add changelogs from release to master

See merge request !80

added empty changelog

See merge request !77

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

See merge request !74

overhaul `TopologySingltonFactory`

See merge request !72

This patch modernizes the code for the `TopologySingletonFactory`. The
main improvement is using `std::unique_ptr` to get rid of `fill_array`,
which is going to be deprecated in the 2.6 release.

bump version to 2.7-git [ci skip]

See merge request !71

[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.