Calculates the volume of intersection divided by the volume of the manifold. For the cube, r is the length of the side. For sphere, r is the radius. For the annulus, r and min_r are the two radii.
calc_overlap_3D(alpha, r = 1, rmin = 0.01, bound = "cube")
volume of overlap
radius of one sphere
radius of second sphere or outer radius of shell or length of cube side
inner radius of shell, only needed if bound=shell
manifold type, options are "cube", "shell", and "sphere"