Returns a GitHub URL for the doc in question (study, collection, ...)

def get_public_url(self, doc_id, branch='master'):
        """Returns a GitHub URL for the doc in question (study, collection, ...)
        """
        name, path_frag = self.get_repo_and_path_fragment(doc_id)
        return 'https://raw.githubusercontent.com/OpenTreeOfLife/' + name + '/' + branch + '/' + path_frag