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 an IndexTree. This tree contains information about uniform and non-uniform sub-structures and about the sizes of this structure. It is implemented for PowerPreBasis and CompositePreBasis and all leaf prebases with a dimension get a default flat IndexTree.

IndexTrees

There are various tree nodes implemented representing all situations I could think of:

• leaf nodes (representing flat containers), with static and dynamic size (StaticFlatIndexTree, DynamicFlatIndexTree)
• uniform trees with all sub-trees of the same type and shape, with static and dynamic degree ([Static]UniformIndexTree)
• non-uniform tree with all sub-trees of either different types or the same types with different shape again with static and dynamic size (TypeUniformIndexTree, StaticTypeUniformIndexTree, NonUniformIndexTree (not implemented), StaticNonUniformIndexTree)

Inner index-tree nodes follow the interface

struct IndexTree
{
  static constexpr bool isUniform = [true|false];     // Whether all children are identical
  static constexpr bool isTypeUniform = [true|false]; // Whether all children have the same type

  template<class Index>
  SubTree operator[](Index i) const;  // return the i-th sub-tree

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

TODO

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