Resolve "MPI error with k-point parallelization"
Fix the issues regarding blacs cleanup:
- call blacs_exit(1) only when running with MPI
- call blacs_gridinit() at least once, as blacs_exit() raises an MPI error in some versions of OpenMPI
- I have checked that my code follows the Octopus coding standards
- I have added tests for all the new features added in this request.
Closes #134 (closed)