docs: def add(self, sim, module, package=None):
          """
          Add simulation to layer.
          """
          super(Simulations, self).add(sim, module, package)
          # only update layer info if it is missing!
          if sim not in self.layer:
              # copy simulation source parameters to :attr:`Layer.layer`
              self.layer[sim] = {'module': module, 'package': package}