title: Replace missing values by merging with another row
body: <p>I read data from different sources into a data.table. Two sources supply different variables for the same time step. </p>
  
  <p>How can I replace the missing variables by the other source (row)? </p>
  
  <pre><code>   Here is a minimal example: 
     (gg = data.table(SourceCode = c(1,1,2,2), time = c(1,2,1,2), LE = c(10,10,NA,NA), R = c(NA,NA,20,30)))
     SourceCode time LE  R
  1:          1    1 10 NA
  2:          1    2 10 NA
  3:          2    1 NA 20
  4:          2    2 NA 30
  
  &gt; # rename SourceCode
  &gt; gg[SourceCode == 1, SourceCode := 2 ]
  &gt; gg
     SourceCode time LE  R
  1:          2    1 10 NA
  2:          2    2 10 NA
  3:          2    1 NA 20
  4:          2    2 NA 30
  </code></pre>
  
  <p>Desired output:</p>
  
  <pre><code>   SourceCode time LE  R
  1:          2    1 10 20
  2:          2    2 10 30
  </code></pre>