title: how to the remove interaction-only columns from sklearn.preprocessing.PolynomialFeatures
body: <p>As per the description given below: 'if an input sample is two dimensional and of the form [a, b], the degree-2 polynomial features are [1, a, b, a^2, ab, b^2].'</p>
  
  <p><a href="http://scikit-learn.org/stable/modules/generated/sklearn.preprocessing.PolynomialFeatures.html#sklearn.preprocessing.PolynomialFeatures.set_params" rel="nofollow noreferrer">sklearn.preprocessing.PolynomialFeatures</a></p>
  
  <p>I need my output ndarray generated to be of the type: [a, b, a^2, b^2]
  I know that <code>poly = PolynomialFeatures(2, include_bias=False)
  poly.fit_transform(X)</code>
  gives the output like : [a, b, a^2, ab, b^2]. But I do not want these intermediate 'ab' type columns that are being generated. How to do that any idea? Or any better API that can be used here?</p>