Test 27, 125 and periodic BCs fail with debugging checks on

If I compile with --enable-debugging-checks --disable-vec, the test27, test125 and testPeriodicBCs all fail with an error message related to the accuracy of the sorted indices. That's not very clean.

@jwillis if you have some spare cycles, could you check whether there is an easy fix there? It could be simply due to the fact that the particles are exactly aligned or something to do with double<->float rounding.