Add CMake PROCESSORS properties

[LecrisUT]

This property will allow to properly parallelize ctest -j calls

[jenkins-cscs]

Can one of the admins verify this patch?

[LecrisUT]

Small note about TEST_MPI_RANKS. It should be fixed to 2 instead of auto in order to better take advantage of ctest parallelization

[alazzaro]

@LecrisUT Thanks for this PR. I like the idea to default the number of ranks to 2. Most of the tests are really intended for max 4 ranks in general... Still, it will be good to provide a way to cap the processors to the max available. I will work on that next week.

[LecrisUT]

Still, it will be good to provide a way to cap the processors to the max available

This is typically done via ctest -j $(nproc)

[hfp]

If this PR targets the next release of CP2K/DBCSR, it would be the right time to finish/conclude it.

[LecrisUT]

Seems like I may need this PR for #759. Tests are timing out due to parallelization there

[alazzaro]

Small note about TEST_MPI_RANKS. It should be fixed to 2 instead of auto in order to better take advantage of ctest parallelization

I've changed the default to be 2, see d8116fd