Full frequency partial directed coherence.

        .. math:: \mathrm{ffPDC}_{ij}(f) =
           \\frac{A_{ij}(f)}{\sqrt{\sum_f A_{:j}'(f) A_{:j}(f)}}

def ffPDC(self):
        """Full frequency partial directed coherence.

        .. math:: \mathrm{ffPDC}_{ij}(f) =
           \\frac{A_{ij}(f)}{\sqrt{\sum_f A_{:j}'(f) A_{:j}(f)}}
        """
        A = self.A()
        return np.abs(A * self.nfft / np.sqrt(np.sum(A.conj() * A, axis=(0, 2),
                                                     keepdims=True)))