docs: def load(self, rel_path=None):
          """
          Add sim_src to layer.
          """
          for k, v in self.layer.iteritems():
              self.add(k, v['module'], v.get('package'))
              filename = v.get('filename')
              path = v.get('path')
              if filename:
                  warnings.warn(DeprecationWarning(SIMFILE_LOAD_WARNING))
                  # default path for data is in ../simulations
                  if not path:
                      path = rel_path
                  else:
                      path = os.path.join(rel_path, path)
                  filename = os.path.join(path, filename)
              self.open(k, filename)