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.