Merge branch 'feature/add-reverse-grid-iterators' into 'master'

Add reverse grid iterators for YaspGrid

This merge request extends the grid interface to support reverse iteration. For this purpose, it adds
* a new type `ReverseIterator` to the `Codim` structs
* a set of `rbegin()` and `rend()` methods (and their equivalents for leaf and level iterators)
* new marker types `Dune::Direction::Forward` and `Dune::Direction::Backward`
* overloads for the entity range generators (`elements(gv)`) etc. that take a direction marker

Right now, only YaspGrid provides support for reverse iteration. Grids without support will simply ignore the request and iterate in forward direction anyway.

See merge request !1