proper testing of mpi-enabled routines in python

The pytest integration of mpi-enable python test is a bit hacky but it works.
The idea is to use a pytest test to spawn processes that will execute the mpi
routines in a parallel environment and reduce the routine result to the pytest
process, which will determine if the routine failed. In our case, the test will
loop through all functions of 'mpi_routines' that start with 'mpi_' and execute
them in an MPI environment.