interaction.h: avoid compiler warning
interaction.h: avoid compiler warning

MrTheodor/csg,votca/csg,votca/csg,junghans/csg,junghans/csg,MrTheodor/csg,votca/csg,junghans/csg,MrTheodor/csg,MrTheodor/csg,votca/csg,junghans/csg,MrTheodor/csg,junghans/csg,junghans/csg