title: Re-shape status columns based on value in another column
body: <p>I have a table that looks like this:</p>
  
  <pre><code>ID    Start_year    Status_2005    Status_2006    Status_2007
   1          2005            GBR            GBR            FRA
   2          2006             NA            FRA            FRA
   3          2007             NA             NA            GBR
   4          2006             NA            UKR            RUS
  </code></pre>
  
  <p>I would like to re-shape the data so that it gives the status in the years subsequent to the start year. So the above, would look like this:</p>
  
  <pre><code>ID     Year_0    Year_1    Year_2
   1        GBR       GBR       GBR
   2        FRA       FRA        NA
   3        GBR        NA        NA
   4        UKR       RUS        NA
  </code></pre>
  
  <p>I have been trying to use tidyverse in R, using gather in combination with "starts_with", and mutate to make the new columns. However, I keep ending up with a single column of "years_since_start_year", and can't work out how to spread this column to make my final table.</p>
  
  <p>Any help much appreciated</p>