How to use the discretize.utils.meshutils function in discretize

X, Y = np.meshgrid(xr, yr)
        Z = A*np.exp(-0.5*((X/b)**2. + (Y/b)**2.)) + 5

        # Create a MAGsurvey
        xyzLoc = np.c_[Utils.mkvc(X.T), Utils.mkvc(Y.T), Utils.mkvc(Z.T)]
        rxLoc = PF.BaseMag.RxObs(xyzLoc)
        srcField = PF.BaseMag.SrcField([rxLoc], param=H0)
        survey = PF.BaseMag.LinearSurvey(srcField)

        # self.mesh.finalize()
        self.mesh = meshutils.mesh_builder_xyz(
            xyzLoc, h, padding_distance=padDist,
            mesh_type='TREE',
        )

        self.mesh = meshutils.refine_tree_xyz(
            self.mesh, topo, method='surface',
            octree_levels=nCpad,
            octree_levels_padding=nCpad,
            finalize=True,
        )

        # Define an active cells from topo
        actv = Utils.surface2ind_topo(self.mesh, topo)
        nC = int(actv.sum())

        # We can now create a susceptibility model and generate data
        # Lets start with a simple block in half-space
        self.model = Utils.ModelBuilder.addBlock(
            self.mesh.gridCC, np.zeros(self.mesh.nC),
            np.r_[-20, -20, -15], np.r_[20, 20, 20], 0.05
        )[actv]

xyzLoc = np.c_[Utils.mkvc(X.T), Utils.mkvc(Y.T), Utils.mkvc(Z.T)]
        Rx = PF.BaseMag.RxObs(xyzLoc)
        srcField = PF.BaseMag.SrcField([Rx], param=H0)
        survey = PF.BaseMag.LinearSurvey(srcField)

        # Create a mesh
        h = [5, 5, 5]
        padDist = np.ones((3, 2)) * 100
        nCpad = [4, 4, 2]

        self.mesh = meshutils.mesh_builder_xyz(
            xyzLoc, h, padding_distance=padDist,
            mesh_type='TREE',
        )

        self.mesh = meshutils.refine_tree_xyz(
            self.mesh, topo, method='surface',
            octree_levels=nCpad,
            octree_levels_padding=nCpad,
            finalize=True,
        )
        # Define an active cells from topo
        actv = Utils.surface2ind_topo(self.mesh, topo)
        nC = int(actv.sum())

        # Convert the inclination declination to vector in Cartesian
        M_xyz = Utils.matutils.dip_azimuth2cartesian(np.ones(nC)*M[0], np.ones(nC)*M[1])

        # Get the indicies of the magnetized block
        ind = Utils.ModelBuilder.getIndicesBlock(
            np.r_[-20, -20, -15], np.r_[20, 20, 20],
            self.mesh.gridCC,