title: Use numpy array / list as index into a string
body: <p>I have a string say <code>s='abcd...z'</code>, and a multi-dimensional numpy array - for example 
  <code>arr = [[2, 5, 11], [1, 3, 9]]</code>. How can I use the array as an index into the string? The output I want is <code>str_arr = [['b', 'e', 'k'],['a', 'c', 'i']]</code>. </p>
  
  <p>I can do this using list comprehensions or for loops, but I am not sure if those are the fastest methods (especially if the array is large). Any vectorized methods? Also looking for something which works not just got 2d arrays but any dimension. </p>