MPI Demo Scripts are outdated

[daehrlich]

For the develop branch, the provided demo script

demo_multivariate_te_mpi.py

erroneously calls the python script using the -m mpi4pyfutures module, which is no longer correct. For instance, the line

srun --nodes 4 -n 4 --mpi=pmi2 python -m mpi4py.futures "IDTxl/demos/demo_multivariate_te_mpi.py" 3

should instead read

srun --nodes 4 -n 4 --mpi=pmi2 python "IDTxl/demos/demo_multivariate_te_mpi.py" 3