calc_pressure_gromacs.sh: make negative pressures work as well
calc_pressure_gromacs.sh: make negative pressures work as well

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