title: Grouping matrix rows in terms of one column
body: <p>First of all it is really hard for me to describe the problem really good but I'll try.</p>
  
  <p>Say that we have matrix A</p>
  
  <pre><code>A = [23 1;
       45 1
       78 1
       86 1
       98 2
       1  2
       23 2
       14 3
       15 4
       85 4]
  </code></pre>
  
  <p>What I want as an output is</p>
  
  <pre><code>    B{1} = [23,45,78,86]
      B{2} = [98,1,23]
      B{3} = [14]
      B{4} = [15,85]
  </code></pre>
  
  <p>Bear in mind that the original A is a huge matrix, and I do not wanna do this with for loops. I would like to use functions that uses parallel processing.</p>