title: Combine all columns of a data.frame in a single one (vector concatenation)
body: <p>I want to convert this data.frame</p>
  
  <pre><code>C1  C2  C3  C4
  q   t   p   k   
  g   l   i   y
  f   d   t   r
  </code></pre>
  
  <p>to a data.frame with <em>a single column</em> with this order of  elements <code>q,t,p,k,g,l,i,y,f,d,t,r</code>. Note that <strong>it is not a concatenation of columns.</strong></p>
  
  <p>Result:</p>
  
  <pre><code>C1
  q
  t
  p
  k
  l
  i
  y
  f
  d
  t
  r
  </code></pre>