Merge branch 'feature/require-cmake-3.0' into 'master'

[CMake] Require at least CMake 3.0

This is a trial balloon to see the fall out. If nobody complains for dune-grid-howto, we can make this change for all core modules.

See [FS#1619](https://gitlab.dune-project.org/flyspray/FS/issues/1619).

See merge request !2

CMakeLists.txt

@@ -2,7 +2,7 @@
 project(dune-grid-howto C CXX)
 
 # general stuff
-cmake_minimum_required(VERSION 2.8.12)
+cmake_minimum_required(VERSION 3.0)
 
 if(NOT (dune-common_DIR OR dune-common_ROOT OR
         "${CMAKE_PREFIX_PATH}" MATCHES ".*dune-common.*"))