find a matrix L which satisfies LLT = YYT.

        Note that L may have fewer columns than Y.

def _get_YYTfactor(self, Y):
        """
        find a matrix L which satisfies LLT = YYT.

        Note that L may have fewer columns than Y.
        """
        N, D = Y.shape
        if (N>=D):
            return Y.view(np.ndarray)
        else:
            return jitchol(tdot(Y))