Add utility functions for recursive traversal of matrices and vectors

This allows

```cpp
recursiveForEachMatrixEntry(matrix, [](auto&& matrix_ij, auto&& i, auto&& j) {
  ...
});

recursiveForEachVectorEntry(vector, [](auto&& vector_i, auto&& i) {
  ...
});
```

Here the container is traversed hierachically and the row and column multi-indices
`i` and `j` are instances of `Dune::Functions::StaticMultiIndex` with appropriate
length depending on the nesting depth.