[Dune] Master requires CMake 3.13

[Christoph Grüninger]

Dear Dune users,

the master of the Dune core modules require CMake 3.13, cf. #200, !809.
For most of you, this should not be a problem. If you don't have a
recent enough CMake you can
- remain before yesterday's commits
- get a package from backports or however it is called for your distribution
- download CMake 3.13 binaries from https://cmake.org/download/ Then you
have to re-run dunecontrol and can add the variable
CMAKE=<pathToNewCmake> You can place the variable definition in front of
your dunecontrol command or into your opts file

Please also update your modules to require CMake 3.13. The CMake code
from dune-common will be executed with the policies from your module.
This mead lead to subtle changes. I didn't find any in my tests. This
could also occur in the CMake code of your module.

Why have we done this? Because it will help us with improving the
CMake-based build system, see #199, and the way we find Python (!806).

Bye
Christoph

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