title: Convert list to data frame while keeping list-element names
body: <p>I have list where the elementnames are ID-tags and contains a vector with numeric values. These are of unequal(!) length.</p>
  
  <p>I want to transform it to a data frame where I have the ID in one column and the numeric values in another column.  E.g.:  </p>
  
  <pre><code>$`1`  
  [1] 1 2   
  $`2`  
  [1] 1 2 3 
  $`3`  
  [1] 1   
  </code></pre>
  
  <p>To:</p>
  
  <pre><code>ID   Obs  
  1    1  
  1    2
  2    1
  2    2
  2    3
  3    1
  </code></pre>