Switch versioning scheme from UG versioning to Dune core versioning

Up to now, the version of the dune-uggrid module was 3.13.0, because that
was the version of UG when we forked it.  However, since dune-uggrid and
dune-grid are not separated by official interfaces, the two modules
practically have to be developed in lock-step.  Therefore starting with
this patch the dune-uggrid module will follow the release cycle and
the version numbering of the Dune core modules.

dune.module

 Module: dune-uggrid
-Version: 3.13.0
+Version: 3.0-git
 Depends: dune-common
 Maintainer: dune-devel@dune-project.org
 Whitespace-Hook: Yes