Update file CMakeLists.txt
requested to merge simon.praetorius/dune-fem-dg:issue/cmake-lists-txt-cmake-minimum-required into master
Summary
Move the command cmake_minimum_required to the first line of the top-level CMakeLists.txt file and update the version number to 3.13. This is required to properly detect dependencies, set cmake policies and... See https://cmake.org/cmake/help/latest/command/cmake_minimum_required.html for some explanations.