docs: def project_meta(self, attributes):
          """ Projects the specified metadata attributes to new region fields
  
          :param attributes: a list of metadata attributes
          :return: a new GDataframe with additional region fields
          """
          if not isinstance(attributes, list):
              raise TypeError('attributes must be a list')
          meta_to_project = self.meta[attributes].applymap(lambda l: ", ".join(l))
          new_regs = self.regs.merge(meta_to_project, left_index=True, right_index=True)
          return GDataframe(regs=new_regs, meta=self.meta)