Draft: Add IndexTree datastructures to represent hierarchic structure of the basis

Summary

This MR implements an idea discussed in a dune-functions developer meeting: Represent the hierarchic structure accessible by multi-indices in a "container descriptor", i.e., a class that is similar to a hierarchic container, with placeholders for the actual data types. This descriptor can be used to construct an actual hierarchic container compatible with the multi-indices provided by the basis.

ContainerDestriptors

There are various destriptors implemented in the namespace Dune::Functions::ContainerDescriptors, following standard containers: Array, Tuple, Vector. Additionally, "uniform"-containers are provided, where all elements (childs) are identical and thus only one of the elements need to be stored. These container destriptors have the prefix Uniform.

The structure of a container-descriptor is a reduced container interface:

struct ContainerDescriptor
{
  template<class Index>
  [SubContainerDescriptor] operator[](Index i) const;  // return the i-th sub-container-descriptor

  [static constexpr] std::size_t size() [const];  // return the number of children
};

With the operator[] you can access the children. The Index type is either an integral value or an integral_constant for tuple nodes.

Size is either a static property, or a runtime value. It might be represented as a static member of integral_constant type, since these have a call operator and even be converted into a std::size_t.

TODO

• Decide whether to name the structure SizeTree or IndexTree --> ContainerDescriptor
• Add changelog entry
• Squash commits
• requires !389 (merged) to be merged and then a rebase to include these commits.
• Update concepts definition

Closes #54