UG grid backend forked into new dune-uggrid module

For many years, the power of the grid manager from the venerable UG finite element code has been available in Dune as the UGGrid grid implementation. For this, you had to build and install UG as a separate library, and dune-grid used it as an external dependencies.

Now, UG has always been much more than a grid data structure, but only the grid data structure was used in dune-grid. To make bugfixes and general maintenance of UG easier, we have now forked UG, and turned the result into a genuine Dune module called dune-uggrid. The new code has already seen considerable cleanup, and nontrivial bugfixes.

In the short run, there are very few changes for users of dune-grid. If you want to use UGGrid with the git master of dune-grid, you now have to install the dune-uggrid module instead of the entire UG software. If present, dunecontrol will find and build it just like any other module.