Add MPI-enabled test cases for statistics
Created by: femtobit
In order to make sure our statistics functions work, I have added a couple of tests to be run with MPI. Adding those tests also uncovered a bug I had overlooked in the MPI stats functions that is now fixed.
The tests are in a separate Test_MPI
folder so that they are not automatically run with the other tests. As a side effect, this also provides a quick way to check that one's own MPI environment is correctly set up.