title: Replace na´s in specific columns by the median of the same columns
body: <p>I would like to replace the na´s in v1 to v4 with the median of the same columns</p>
  
  <p>Here are some example data </p>
  
  <pre><code>id &lt;- c(1,2,3,4)
  v1 &lt;- c(1,3,0,2)
  v2 &lt;- c(NA,1,NA,2)
  v3 &lt;- c(2,4,1,2)
  v4 &lt;- c(NA,1,0,2)
  v5 &lt;- c(5,1,NA,2)
  v6 &lt;- c(7,1,9,NA)
  
  df &lt;- data.frame(id, v1, v2, v3,v4,v5,v6)
  df_pre &lt;- df %&gt;% group_by(id) %&gt;% mutate(Median_v1_v4 = median(c(v1,v2,v3,v4), na.rm=TRUE))
  </code></pre>
  
  <p>This is what data looks like now: </p>
  
  <pre><code>id v1 v2 v3 v4 v5 v6  Median_v1_v4
   1  1 NA  2 NA  5  7          1.5
   2  3  1  4  1  1  1          2.0
   3  0 NA  1  0 NA  9          0.0
   4  2  2  2  2  2 NA          2.0
  </code></pre>
  
  <p>This is what i want the data to look like </p>
  
  <pre><code>id v1  v2 v3  v4 v5 v6 Median_v1_v4
  1  1 1.5  2 1.5  5  7          1.5
  2  3 1.0  4 1.0  1  1          2.0
  3  0 0.0  1 0.0 NA  9          0.0
  4  2 2.0  2 2.0  2 NA          2.0
  </code></pre>