Fixes so that tests compile with g++

src/single_io.c

src/single_io.h

tests/test125cells.c

tests/test27cells.c

tests/testActivePair.c

tests/testDump.c

tests/testFFT.c

tests/testLogger.c

tests/testPeriodicBC.c

tests/testPotentialPair.c

tests/testSPHStep.c

tests/testTimeIntegration.c