Added the Mf.test_bug_24131_missing_interaction system test to catch another problem.
This is part of bug #24131 (https://gna.org/bugs/?24131), the BMRB export failure with the
SpinContainer object having no s2 value. However the previous fix of skipping deselected spins
introduced a new problem of relax still searching for the interatomic interactions for that
deselected spin.