title: How to pivot a dataframe so the column labels become elements in the dataframe
body: <p>I have a dataframe that looks like this:</p>
  <pre><code>        Date        a45     a52     b28     ...     c75     c77     c78     
  0       1/1/1981    -9999   9.473   8.987   ...     12.689  12.199  11.928 
  1       1/2/1981    -9999   5.832   7.401   ...     14.001  13.327  13.323 
  2       1/3/1981    -9999   4.248   5.025   ...     13.045  11.538  11.472 
  3       1/4/1981    -9999   6.049   8.592   ...     15.421  14.208  14.319 
  4       1/5/1981    -9999   -2.755  1.395   ...     13.289  11.701  11.684 
  ...     ...         ...     ...     ...     ...     ...     ...     ...
  2127    7/27/2020   -9999   -1.708  1.603   ...     13.123  11.085  11.113 
  2128    7/28/2020   -9999   1.623   4.770   ...     15.262  14.132  14.060 
  2129    7/29/2020   -9999   1.042   3.118   ...     11.848  10.227  10.168 
  2130    7/30/2020   -9999   3.716   5.778   ...     11.192  10.389  10.464 
  2131    7/31/2020   -9999   2.019   6.944   ...     11.211  11.018  11.063 
  </code></pre>
  <p>My goal is to pivot it but I can't figure it out. I want it to look like this</p>
  <pre><code>Date        site    temp
  1/1/1981    a45     -9999
  1/2/1981    a45     -9999
  1/3/1981    a45     -9999
  1/4/1981    a45     -9999
  1/5/1981    a45     -9999
  ...         ...     ...
  7/27/2020   c78     11.113 
  7/28/2020   c78     14.060 
  7/29/2020   c78     10.168 
  7/30/2020   c78     10.464 
  7/31/2020   c78     11.063 
  </code></pre>
  <p>Any help would be greatly appreciated.</p>