Given an orthogonal box, oriented along the cartesian axes with lengths 2*rx, 2*ry and 2*rz and centred on the origin, what parameters must we pass to supercell to make a system big enough from our original cell defined by lattice for the box to fit inside?
The required supercell parameters are returned in Na, Nb, Nc respectively. If, e.g. Na = 1 then we don't need to supercell in the a direction. Na > 1 means we must supercell by Na times in the direction.