title: Eliminate NAs within each group
body: <p>I have ended up with rows that have NAs that correspond to a column that was deleted and I want to get rid of those NAs. This would turn the example df below from three rows to one row and leave me one set of variables for each group. </p>
  
  <pre><code>df &lt;- data.frame(group = c("a", "a", "a"), var1 = c(1, NA, NA), var2 = c(NA, 4, NA), var3 = c(NA, NA, 
  2), var4 = c(1, NA, NA), var5 = c(NA, 4, NA), var6 = c(NA, NA, 2), var7 = c(1, NA, NA), 
  var8 = c(NA, 4, NA), var3 = c(NA, NA, 2))    
  </code></pre>
  
  <p>I know how to get rid of whole rows with NAs or columns with NAs, but am not sure how to collapse this.  </p>