🔧 Update min cmake version to 3.1

doc/developers/configuration.rst

@@ -10,7 +10,7 @@ below::
 
   name: numgrid
 
-  min_cmake_version: 2.8
+  min_cmake_version: 3.1
 
   default_build_type: release
 
@@ -75,7 +75,7 @@ Minimal example
 As a minimal example we take an ``autocmake.yml`` which only contains::
 
   name: minime
-  min_cmake_version: 2.8
+  min_cmake_version: 3.1
   language: C
 
 If you don't have the ``update.py`` script yet, you need to fetch it from the web::

doc/developers/example.rst

@@ -54,7 +54,7 @@ Now we edit ``cmake/autocmake.yml`` to look like this::
 
   name: hello
 
-  min_cmake_version: 2.8
+  min_cmake_version: 3.1
 
   language:
     - Fortran

doc/developers/faq.rst

@@ -73,7 +73,7 @@ Yes you can do that in ``autocmake.yml``. Here we for instance change the name t
   :emphasize-lines: 4
 
   name: myproject
-  min_cmake_version: 2.8
+  min_cmake_version: 3.1
   default_build_type: release
   setup_script: configure
   language: