[dune-functions] TypeTree now works with dune-functions again

[Steffen Müthing]

Hi everyone,

I finally cleaned up the HybridTreePath stuff in dune-typetree (that escalated into something
a little larger than expected…).

There is now a HybridTreePath in TypeTree, which is a wrapper around std::tuple (I didn’t just
alias or inherit for a number of reasons). I replaced all occurrences of Carsten’s interim implementation
that I could find. When trying to compile everything, I noticed a surprising amount of breakage in our
tests… is it just me, or do we have to start fighting the bitrot? ;-)

I also really liked Carsten’s idea of having an index_constant that’s a template alias for an
integral_constant<std::size_t,…>, so I put that into TypeTree and started to use it extensively. There’s
also a standard-compliant reimplementation of the integer_range and index_range functionality in
Dune::TypeTree::Std (BTW, it’s *really* bad that we have something not quite correct in Dune::Std).

The pre-made index objects now live in Dune::TypeTree::Indices (I thought the „Static“ was a little redundant).
I really like the idea of those constants and would like to see them in dune-common after the release of 2.4.

There is now a new API for accessing children in TypeTree: to get a child, just write

Dune::TypeTree::child(node,indices...);

Indices can be either a combination of std::size_t and index_constant, a TreePath (fully static) or a HybridTreePath.
For convenience (to avoid having to qualify child() all the time), there are also new member functions, so you
can also write

node.child(indices…);

Beware: Doing so is problematic in generic code that expects to call node.child(treePath) with an empty TreePath
on a leaf node. That only works with the freestanding function - the member function cannot specify the correct
return type in that case. So generic code (like the stuff in defaultnodetorangemap.hh) will have to use the freestanding
functions.

The type of a child can be extracted using one of two template aliases:

using ChildType = Dune::TypeTree::Child<Node,0,3,_2>; // this accepts both integer literals and the predefined index_constants
using ChildType = Dune::TypeTree::ChildForTreePath<Node,TreePath>; // the path can be either TreePath or HybridTreePath

I quite like the way it works now, but if you have any suggestions for improvements, shoot!

Steffen
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