Merge branch 'issue/restrict_mpi_version' into 'master'

require at least MPI >= 3.0

See merge request !891

CHANGELOG.md

 # Master (will become release 2.8)
 
+- Set minimal required MPI version in cmake to >= 3.0.
+
 - Previous versions of dune-common imported `std::shared_ptr` and `std::make_shared`
   into the `Dune` namespace.  dune-common-2.8 stops doing that.
 

cmake/modules/DuneMPI.cmake

@@ -21,7 +21,7 @@
 #
 
 
-find_package(MPI)
+find_package(MPI 3.0 COMPONENTS C)
 
 # text for feature summary
 set_package_properties("MPI" PROPERTIES