title: Combining rows of the same key into single array
body: <p>I have a pandas dataframe as follows:</p>
  
  <pre><code>    error
   0: [[0.1,0.4,-0.3]]
   1: [[-0.6,-0.3,0.2]]
   .
   .
   .
   99: [[0.4,-0.7,0.1]]
  </code></pre>
  
  <p>I would like to combine all values into a single array like this:</p>
  
  <pre><code> [0.1,0.4,-0.3,-0.6,-0.3,0.2,...,0.4,-0.7,0.1]
  </code></pre>
  
  <p>Is there a fast way to do this using pandas or do I need to iterate over the data and build the array "manually" ?
  The data order, in this case, is not important.</p>
  
  <p>In a more general case, how to combine arrays that don't have the same size (e.g. row 0 contains an array of 3 elements, row 1 contains an array of 6 elements,etc...) ?</p>