title: Add a constant value to all rows in a dataframe
body: <p>I want to add the number <code>1</code> to all the rows and columns of dataframe. Here's the dataframe:</p>
  
  <pre><code>df &lt;- structure(list(units = c(422, 188, 314, 197, 2825, 3210, 450, 
  117, 291, 161, 113, 259), quantity = c(361, 163, 298, 74, 1997, 
  2803, 164, 62, 203, 140, 115, 269), KP = c(282, 194, 295, 102, 
  1772, 1610, 151, 295, 126, 157, 191, 233), level = c(215, 62, 
  215, 78, 1646, 635, 128, 65, 126, 402, 552, 281)), .Names = c("units", 
  "quantity", "KP", "level"), row.names = c(NA, 12L), class = "data.frame")
  </code></pre>
  
  <p>which looks like:</p>
  
  <pre><code>   units quantity   KP level
  1    422      361  282   215
  2    188      163  194    62
  3    314      298  295   215
  4    197       74  102    78
  5   2825     1997 1772  1646
  6   3210     2803 1610   635
  7    450      164  151   128
  8    117       62  295    65
  9    291      203  126   126
  10   161      140  157   402
  11   113      115  191   552
  12   259      269  233   281
  </code></pre>
  
  <p>And my target is:</p>
  
  <pre><code>   units quantity   KP level
  1    423      362  283   216
  2    189      164  195    63
  3    315      299  296   216
  4    198       75  104    79
  ...
  etc
  </code></pre>
  
  <p>Thank yall for your help..</p>