Get the a singleton API resource.

        Some Helium API resources are singletons. The authorized user
        and organization for a given API key are examples of this.

        .. code-block:: python

            authorized_user = User.singleton(session)

        will retrieve the authorized user for the given
        :class:`Session`

        Keyword Args:

            include: Resource classes to include

def singleton(cls, session, include=None):
        """Get the a singleton API resource.

        Some Helium API resources are singletons. The authorized user
        and organization for a given API key are examples of this.

        .. code-block:: python

            authorized_user = User.singleton(session)

        will retrieve the authorized user for the given
        :class:`Session`

        Keyword Args:

            include: Resource classes to include

        """
        params = build_request_include(include, None)
        url = session._build_url(cls._resource_path())
        process = cls._mk_one(session, singleton=True, include=include)
        return session.get(url, CB.json(200, process), params=params)