title: Model matrix with all pairwise interactions between columns
body: <p>Let's say that I have a numeric data matrix with columns <code>w, x, y, z</code> and I also want to add in the columns that are equivalent to <code>w*x, w*y, w*z, x*y, x*z, y*z</code> since I want my covariate matrix to include all pairwise interactions.</p>
  
  <p>Is there a clean and effective way to do this?</p>