Merge branch 'feature/stop-finding-old-superlu' into 'master'

Stop finding old SuperLU versions.

Previously CMake pretended to have found a valid
SuperLU version for DUNE even if the version was
too old (i.e. < 4). People having such an old versions
only noticed this via later compile errors of binaries
using SuperLU. That is far too late.

With this patch we introduce a compile check into the
CMake module that fails for old unsupported versions.
In that case CMake will not find SuperLU.

See merge request !52