title: In python, reading multiple CSV's, with different headers, into one dataframe
body: <p>I have dozens of csv files with similar (but not always exactly the same) headers. For instance, one has:</p>
  
  <pre><code>Year Month Day Hour Minute Direct Diffuse D_Global D_IR Zenith Test_Site
  </code></pre>
  
  <p>One has: </p>
  
  <pre><code>Year Month Day Hour Minute Direct Diffuse2 D_Global D_IR U_Global U_IR Zenith Test_Site
  </code></pre>
  
  <p>(Notice one lacks "U_Global" and "U_IR", the other has "Diffuse2" instead of "Diffuse")</p>
  
  <p>I know how to pass multiple csv's into my script, but how do I have the csv's only pass values to columns in which they currently have values? And perhaps pass "Nan" to all other columns in that row.</p>
  
  <p>Ideally I'd have something like:</p>
  
  <pre><code>'Year','Month','Day','Hour','Minute','Direct','Diffuse','Diffuse2','D_Global','D_IR','U_Global','U_IR','Zenith','Test_Site'
  1992,1,1,0,3,-999.00,-999.00,"Nan",-999.00,-999.00,"Nan","Nan",122.517,"BER"
  2013,5,30,15,55,812.84,270.62,"Nan",1078.06,-999.00,"Nan","Nan",11.542,"BER"
  2004,9,1,0,1,1.04,79.40,"Nan",78.67,303.58,61.06,310.95,85.142,"ALT"
  2014,12,1,0,1,0.00,0.00,"Nan",-999.00,226.95,0.00,230.16,115.410,"ALT"
  </code></pre>
  
  <p>The other caveat, is that this dataframe needs to be appended to. It needs to remain as multiple csv files are passed into it. I think I'll probably have it write out to it's own csv at the end (it's eventually going to NETCDF4).</p>