Periodic boundary conditions with CMFD and OpenMP threads

[GiudGiud]

The CMFD fails to converge for the test case in test_cmfd_print_ratios.py, but only when all the following are true

• CMFD is used
• openmp threads are used, 2 or more
• periodic boundary conditions are used

This is definitely a bug since using a single thread works.
Periodic boundary conditions have gone untested for a long time, it's not clear whether this is a new bug or it's simply been uncovered by the fixes in #463