title: Spark Unique pair in cartesian product
body: <p>I have this:</p>
  
  <pre><code>In [1]:a = sc.parallelize([a,b,c])
  In [2]:a.cartesian(a).collect()
  Out[3]: [(a, a), (a, b), (a, c), (b, a), (c, a), (b, b), (b, c), (c, b), (c, c)]
  </code></pre>
  
  <p>I want the following result:</p>
  
  <pre><code>In [1]:a = sc.parallelize([1,2,3])
  In [2]:a.cartesianMoreInteligent(a).collect()
  Out[3]: [(a, a), (a, b), (a, c), (b, b), (b, c), (c, c)]
  </code></pre>
  
  <p>Because my calculus return a symetrical matrix (correlation). 
  What is the best way to achieve this ? (No loop)
  With a, b and c can be anything, even tuple.</p>