docs: def create_control_volumes(self):
          """Compute the control volumes of all nodes in the mesh.
          """
          self.control_volumes = numpy.zeros(len(self.node_coords), dtype=float)
          for k, cell in enumerate(self.cells):
              node_ids = cell["nodes"]
              self.control_volumes[node_ids] += 0.5 * self.cell_volumes[k]
  
          # Sanity checks.
          sum_cv = sum(self.control_volumes)
          sum_cells = sum(self.cell_volumes)
          alpha = sum_cv - sum_cells
          assert abs(alpha) < 1.0e-6
          return