title: Group values from one numpy array based on values from another
body: <p>I have had a look through SO to see if I can find the answer to my query but I think my knowledge of numpy at this point is too low to know if I have found it or not!</p>
  <p>Given two array of the same shape, I would like to group values from array B based on shared values in group A.</p>
  <p>For example</p>
  <pre><code>arraya = [1,1,1,1,2,2,2,2,3,3,3,3]
  arrayb = [45,56,75,34,657,67,88,32,56,3,45,8]
  </code></pre>
  <p>I essentially want out of this three arrays - 1, 2 and 3 based on the unique values from arraya - they would then contain the values in the same position.</p>
  <pre><code>array1 = [45,56,75,34]
  array2 = [657,67,88,32]
  array3 = [56,3,45,8]
  </code></pre>
  <p>Any pointers would be great! Thank you SO community.</p>