Add a vector factory based on the container descriptors

Summary

This MR adds a factory for istl containers capable of storing the coefficients of a basis with multi-indices. It is based on the recently introduced container-descriptor facilities, which define the shape of a possible nested container.

The MR provides the following function:

#include <dune/functions/backends/istlvectorfactory.hh>

template<class T = double, class ContainerDescriptor>
auto istlVectorFactory (const ContainerDescriptor& tree);

Example

The container descriptor is provided by a basis, e.g.,

using namespace Dune::Functions::BasisFactory;
auto basis = makeBasis(grid.leafGridView(), 
  composite(power<2>(lagrange<2>(), blockedLexicographic()), lagrange<1>()));
auto u_basis = subspaceBasis(basis, _0);
auto p_basis = subspaceBasis(basis, _1);

// create a vector capable of storing the coefficients for that basis
using Dune::Functions::istlVectorFactory;
auto coefficients = istlVectorFactory<double>(basis.preBasis().containerDescriptor());

using Dune::Functions::interpolate;
interpolate(u_basis, coefficients, [](auto const& x) { return Dune::FieldVector<double,2>{2.0,2.0}; });
interpolate(p_basis, coefficients, [](auto const& x) { return 1.0; });

ToDo

• Requires core/dune-istl!563 (merged) to be merged before.