docs: def _update(self, layer=None):
          """
          Update layers in model.
          """
          meta = getattr(self, ModelBase._meta_attr)
          if not layer:
              layers = self.layers
          else:
              # convert non-sequence to tuple
              layers = _listify(layer)
          for layer in layers:
              # relative path to layer files from model file
              path = os.path.abspath(os.path.join(meta.modelpath, layer))
              getattr(self, layer).load(path)