title: Reshape wide format, to multi-column long format
body: <p>I want to reshape a wide format dataset that has multiple tests which are measured at 3 time points:</p>
  
  <pre><code>   ID   Test Year   Fall Spring Winter
      1   1   2008    15      16      19
      1   1   2009    12      13      27
      1   2   2008    22      22      24
      1   2   2009    10      14      20
      2   1   2008    12      13      25
      2   1   2009    16      14      21
      2   2   2008    13      11      29
      2   2   2009    23      20      26
      3   1   2008    11      12      22
      3   1   2009    13      11      27
      3   2   2008    17      12      23
      3   2   2009    14      9       31
  </code></pre>
  
  <p>into a data set that separates the tests by column but converts the measurement time into long format, for each of the new columns like this:</p>
  
  <pre><code>    ID  Year    Time        Test1 Test2
      1   2008    Fall        15      22
      1   2008    Spring      16      22
      1   2008    Winter      19      24
      1   2009    Fall        12      10
      1   2009    Spring      13      14
      1   2009    Winter      27      20
      2   2008    Fall        12      13
      2   2008    Spring      13      11
      2   2008    Winter      25      29
      2   2009    Fall        16      23
      2   2009    Spring      14      20
      2   2009    Winter      21      26
      3   2008    Fall        11      17
      3   2008    Spring      12      12
      3   2008    Winter      22      23
      3   2009    Fall        13      14
      3   2009    Spring      11      9
      3   2009    Winter      27      31
  </code></pre>
  
  <p>I have unsuccessfully tried to use reshape and melt. Existing posts address transforming to single column outcome. </p>