* cray-libsci: only be a provider for scalapack with +mpi
If a package explicitly links the scalapack provider we might otherwise end up with different variants of libsci being linked: the explicitly linked one and the one added by the Cray compiler wrappers.
* cp2k: require cray-libsci+openmp with +openmp for consistency
otherwise we might get 2 different libsci linked: one explicitly, the other one via the Cray compiler wrappers, leading at least to segfaults during cleanup
* cp2k: depend on cray-fftw+openmp with +openmp