Tools to average pressures in 3D grid over different coordinate systems. Originally used in Ollila et al. (2009) 3D Pressure Field in Lipid Membranes and Membrane-Protein Complexes. Phys. Rev. Lett. 102: 078101 [DOI: 10.1103/PhysRevLett.102.078101] and shared http://md.chem.rug.nl/cgmartini/index.php/3d.