How to use the pygalmesh.Ball function in pygalmesh

def test_ball():
    s = pygalmesh.Ball([0.0, 0.0, 0.0], 1.0)
    mesh = pygalmesh.generate_mesh(s, cell_size=0.2, verbose=False)

    assert abs(max(mesh.points[:, 0]) - 1.0) < 0.02
    assert abs(min(mesh.points[:, 0]) + 1.0) < 0.02
    assert abs(max(mesh.points[:, 1]) - 1.0) < 0.02
    assert abs(min(mesh.points[:, 1]) + 1.0) < 0.02
    assert abs(max(mesh.points[:, 2]) - 1.0) < 0.02
    assert abs(min(mesh.points[:, 2]) + 1.0) < 0.02

    vol = sum(helpers.compute_volumes(mesh.points, mesh.cells["tetra"]))
    assert abs(vol - 4.0 / 3.0 * numpy.pi) < 0.15
    return

def generate_mesh():
    """Generates a fairly large mesh.
    """
    # import meshzoo
    # points, cells = meshzoo.rectangle(nx=300, ny=300)
    # return meshio.Mesh(points, {"triangle": cells})
    if os.path.isfile("cache.xdmf"):
        mesh = meshio.read("cache.xdmf")
    else:
        s = pygalmesh.Ball([0, 0, 0], 1.0)
        mesh = pygalmesh.generate_mesh(s, cell_size=2.0e-2, verbose=True)
        # mesh = pygalmesh.generate_mesh(s, cell_size=1.0e-1, verbose=True)
        mesh.cells = {"tetra": mesh.cells["tetra"]}
        mesh.point_data = []
        mesh.cell_data = {"tetra": {}}
        mesh.write("cache.xdmf")
    print(mesh)
    return mesh

def test_balls_union():
    radius = 1.0
    displacement = 0.5
    s0 = pygalmesh.Ball([displacement, 0, 0], radius)
    s1 = pygalmesh.Ball([-displacement, 0, 0], radius)
    u = pygalmesh.Union([s0, s1])

    a = numpy.sqrt(radius ** 2 - displacement ** 2)
    edge_size = 0.1
    n = int(2 * numpy.pi * a / edge_size)
    circ = [
        [0.0, a * numpy.cos(i * 2 * numpy.pi / n), a * numpy.sin(i * 2 * numpy.pi / n)]
        for i in range(n)
    ]
    circ.append(circ[0])

    mesh = pygalmesh.generate_mesh(
        u, feature_edges=[circ], cell_size=0.15, edge_size=edge_size, verbose=False
    )

import os

import meshio
import matplotlib.pyplot as plt
import numpy
import pygalmesh

s = pygalmesh.Ball([0, 0, 0], 1.0)
mesh = pygalmesh.generate_mesh(s, cell_size=3.0e-2, verbose=True)
mesh.cells = {"tetra": mesh.cells["tetra"]}
mesh.point_data = {}
mesh.cell_data = {}

print("num points: {}".format(mesh.points.shape[0]))

formats = {
    "VTU (binary)": (lambda f, m: meshio.vtu.write(f, m, binary=True), ["out.vtu"]),
    "VTU (ASCII)": (lambda f, m: meshio.vtu.write(f, m, binary=False), ["out.vtu"]),
    "VTK (binary)": (lambda f, m: meshio.vtk.write(f, m, binary=True), ["out.vtk"]),
    "VTK (ASCII)": (lambda f, m: meshio.vtk.write(f, m, binary=False), ["out.vtk"]),
    "Gmsh 4.1 (binary)": (
        lambda f, m: meshio.gmsh.write(f, m, binary=True),
        ["out.msh"],
    ),

def test_sphere():
    radius = 1.0
    s = pygalmesh.Ball([0.0, 0.0, 0.0], radius)
    mesh = pygalmesh.generate_surface_mesh(
        s, angle_bound=30, radius_bound=0.1, distance_bound=0.1, verbose=False
    )

    tol = 1.0e-2
    assert abs(max(mesh.points[:, 0]) - radius) < tol
    assert abs(min(mesh.points[:, 0]) + radius) < tol
    assert abs(max(mesh.points[:, 1]) - radius) < tol
    assert abs(min(mesh.points[:, 1]) + radius) < tol
    assert abs(max(mesh.points[:, 2]) - radius) < tol
    assert abs(min(mesh.points[:, 2]) + radius) < tol

    areas = helpers.compute_triangle_areas(mesh.points, mesh.cells["triangle"])
    surface_area = sum(areas)
    assert abs(surface_area - 4 * numpy.pi * radius ** 2) < 0.1
    return

def test_balls_intersection():
    radius = 1.0
    displacement = 0.5
    s0 = pygalmesh.Ball([displacement, 0, 0], radius)
    s1 = pygalmesh.Ball([-displacement, 0, 0], radius)
    u = pygalmesh.Intersection([s0, s1])

    a = numpy.sqrt(radius ** 2 - displacement ** 2)
    edge_size = 0.1
    n = int(2 * numpy.pi * a / edge_size)
    circ = [
        [0.0, a * numpy.cos(i * 2 * numpy.pi / n), a * numpy.sin(i * 2 * numpy.pi / n)]
        for i in range(n)
    ]
    circ.append(circ[0])

    mesh = pygalmesh.generate_mesh(
        u, feature_edges=[circ], cell_size=0.15, edge_size=edge_size, verbose=False
    )

    assert abs(max(mesh.points[:, 0]) - (radius - displacement)) < 0.02

def test_balls_intersection():
    radius = 1.0
    displacement = 0.5
    s0 = pygalmesh.Ball([displacement, 0, 0], radius)
    s1 = pygalmesh.Ball([-displacement, 0, 0], radius)
    u = pygalmesh.Intersection([s0, s1])

    a = numpy.sqrt(radius ** 2 - displacement ** 2)
    edge_size = 0.1
    n = int(2 * numpy.pi * a / edge_size)
    circ = [
        [0.0, a * numpy.cos(i * 2 * numpy.pi / n), a * numpy.sin(i * 2 * numpy.pi / n)]
        for i in range(n)
    ]
    circ.append(circ[0])

    mesh = pygalmesh.generate_mesh(
        u, feature_edges=[circ], cell_size=0.15, edge_size=edge_size, verbose=False
    )