Merge branch 'doc/update-version' into 'master'

ref:staging/dune-typetree The version was still stuck at 2.6.0-rc1.

See merge request [!50]

  [!50]: gitlab.dune-project.org/staging/dune-typetree/merge_requests/50

Merge branch 'feature/make-child-template-invalid-if-child-not-found' into 'master'

ref:staging/dune-typetree The type alias Child<Node,...> used to simply be
void if the child type could not be extracted. That is rather unhelpful, as it
makes it impossible to use in SFINAE.

This patch changes the behavior by making it impossible to instantiate the
type alias if there is no valid child.

See merge request [!49]

  [!49]: gitlab.dune-project.org/staging/dune-typetree/merge_requests/49

The type alias `Child<Node,...>` used to simply be `void` if the child type
could not be extracted. That is rather unhelpful, as it makes it impossible to
use in SFINAE.

This patch changes the behavior by making it impossible to instantiate the type
alias if there is no valid child.

Merge branch 'ci/centralize-config' into 'master'

See merge request [!48]

  [!48]: Nonestaging/dune-typetree/merge_requests/48

Closes #8

See merge request !47

See merge request !46

Implement helper makeTreeContainer() to construct container matching the tree structure

See merge request !37

Revert "Merge branch 'cherry-pick-9cf4ae48' into 'releases/2.6'"

See merge request !43

This reverts merge request !42

[LeafNode] Add empty child() method for consistency

See merge request !41

[doc] Fix doc in childextraction.hh

See merge request !38

This implements a helper function makeTreeContainer() which creates
a nested container matching the structure of a given tree. PowerNodes
are represented by std::array while CompositeNodes are represented
by Dune::TupleVector. The data (and its type) stored for the leafs
is created by a user-provided callback.

For convenience, the nested container is not returned directly but
wrapped such that you can use operator[](HybridTreePath) to access
it's entries.

This is helpfull if you want to associate date to the leaf nodes
but want to avoid having to create a full type tree. E.g. for
implementing a local grid function spanned by an ansatz tree
of local finite elements, each leaf evaluates its basis functions
into an std::vector. This is better created only once to avoid
constly reallocation. The new utility allows to conveneintly store
and access one std::vector for each leaf node.

[ci] Switch to Heidelberg buildslave

See merge request !35

Update stated compiler requirements

See merge request !33

Bring them in line with what we actually require.

Feature/update changelog

See merge request !29

Add overload for dynamic child traversal of PowerNodes

See merge request !30

add missing include for `Dune::Std::bool_constant`

See merge request !32

Fix HybridTreePath element access with GCC

See merge request !31

GCC fails to capture this in the lambda closure of a lambda defined in a member function unless it
is explicitly mentioned.

For now, just check that element access works.

Using Dune::Hybrid allows to avoid code duplication and
use the same loop to traverse statically or dynamically.
Only the type of the provided range size for the loop
has to be switched.

We require that `visitor.in(node,tp)` can always be instantiated.
If this cotains code that requires `node.CHILDREN`>1 to compile,
this has to be guarded internally in the `visitor`.