title: More efficient way to do `Q.slice(a).col(b)` using Armadillo
body: <p>Is there a more efficient way to extract a column from a cube in Armadillo than <code>Q.slice(a).col(b)</code>?</p>
  
  <p>Something like <code>tube</code>, but for the last 2 dimensions instead of the first 2.</p>