code: def cable_length(self):
      
      v1 = self.vertices[self.edges[:,0]]
      v2 = self.vertices[self.edges[:,1]]
  
      delta = (v2 - v1)
      delta *= delta
      dist = np.sum(delta, axis=1)
      dist = np.sqrt(dist)
  
      return np.sum(dist)
docs: Returns cable length of connected skeleton vertices in the same
      metric that this volume uses (typically nanometers).