title: Reshaping multiple sets of measurement columns (wide format) into single columns (long format)
body: <p>I have a dataframe in a wide format, with repeated measurements taken within different date ranges. In my example there are three different periods, all with their corresponding values. E.g. the first measurement (<code>Value1</code>) was measured in the period from <code>DateRange1Start</code> to <code>DateRange1End</code>: </p>
  
  <pre><code>ID DateRange1Start DateRange1End Value1 DateRange2Start DateRange2End Value2 DateRange3Start DateRange3End Value3
  1 1/1/90 3/1/90 4.4 4/5/91 6/7/91 6.2 5/5/95 6/6/96 3.3 
  </code></pre>
  
  <p>I'm looking to reshape the data to a long format such that the DateRangeXStart and DateRangeXEnd columns are grouped,. Thus, what was 1 row in the original table becomes 3 rows in the new table:</p>
  
  <pre><code>ID DateRangeStart DateRangeEnd Value
  1 1/1/90 3/1/90 4.4
  1 4/5/91 6/7/91 6.2
  1 5/5/95 6/6/96 3.3
  </code></pre>
  
  <p>I know there must be a way to do this with <code>reshape2</code>/<code>melt</code>/<code>recast</code>/<code>tidyr</code>, but I can't seem to figure it out how to map the multiple sets of measure variables into single sets of value columns in this particular way.</p>