Keep only the specified sources in the decomposition.

def keep_sources(self, keep):
        """Keep only the specified sources in the decomposition.
        """
        if self.unmixing_ is None or self.mixing_ is None:
            raise RuntimeError("No sources available (run do_mvarica first)")
        n_sources = self.mixing_.shape[0]
        self.remove_sources(np.setdiff1d(np.arange(n_sources), np.array(keep)))
        return self