update to DUNE v2.5.1

This makes DORiE compatible to DUNE v2.5.1, which is contained in the releases/2.5 branches as before.

This fixes #27 (closed), #24 (closed), #16 (closed).

• The dune-python package was removed as dependency, since its functions have been moved to dune-common. This changed some of the CMake interfaces for Python calls.
• dune-testtools version has been bumped to master, since its releases/2.5 branch has not been updated yet.
• legacy UG grid is not supported anymore
• recommending Python 3 for all setups
• revisited README