Center `X` in PCA space.

def center(self, X):
        """
        Center `X` in PCA space.
        """
        X = X.copy()
        inan = numpy.isnan(X)
        if self.mu is None:
            X_ = numpy.ma.masked_array(X, inan)
            self.mu = X_.mean(0).base
            self.sigma = X_.std(0).base
        reduce(lambda y,x: setitem(x[0], x[1], x[2]), zip(X.T, inan.T, self.mu), None)
        X = X - self.mu
        X = X / numpy.where(self.sigma == 0, 1e-30, self.sigma)
        return X