title: Duplicate element in list x amount of times based off another list
body: <p>I have a list of numbers and a list of strings <br></p>
  <pre><code>a = ['2', '2', '3', '4']
  b = [' ', 'A', 'B', 'C']
  </code></pre>
  <p>I want to duplicate each string in <code>b</code> <code>a</code> times making my output look like:</p>
  <pre><code>[' ', ' ', 'A', 'A', 'B', 'B', 'B', 'C', 'C', 'C', 'C']
  </code></pre>
  <p>I've tried using <code>map</code> a bunch of different ways. Also would this be easier to do if I just switched the lists to <code>numpy</code> arrays and dealing it with that package.</p>