Update `B`.

def _update_B(self):
        """Update `B`."""
        for param in self.freeparams:
            if param == 'mu':
                continue
            paramval = getattr(self, param)
            if isinstance(paramval, float):
                self.B[param] = broadcastMatrixMultiply(self.Ainv,
                        broadcastMatrixMultiply(self.dPrxy[param], self.A))
            else:
                assert isinstance(paramval, numpy.ndarray) and paramval.ndim == 1
                for j in range(paramval.shape[0]):
                    self.B[param][j] = broadcastMatrixMultiply(self.Ainv,
                            broadcastMatrixMultiply(self.dPrxy[param][j],
                            self.A))