Add reverse grid iterators for UGGrid

This is the continuation of the work done in !1 (merged). But here it is done for UGGrid because we also want a matrix-free SSOR preconditioner on this grid. It adds

1. Change to self-standing typedefs for the ReverseIterator that do not simply pass to the forward iterators both on the level and leaf grid view. Also introduce the rbegin and rend methods in the two grid views.

2. Introduce reverse iterators both for level and leaf. Those two implementations exchange

   • First by Last
   • succ by pred compared to the corresponding forward versions.

3. Add wrappers for the pred and all variants of Last methods to access what we wish in dune-uggrid.