title: How to convert column number into a letter
body: <p>I am writing a query formula via script. I have a dialogue which I would like to list all the available columns that have data in them. I can easily get the number of columns that the sheet has using <code>sheet.getLastColumn();</code>, but how can I easily convert that to the column format A,B,C...AA,BB,CC,... ?</p>
  
  <p>So if the number of columns is 5 I am trying to get an array [A,B,C,D,E]  If it is 30 then I would expect an array of [A,B,C,...,CC].</p>
  
  <p>Not sure why it is not letting me post, but I thought I would write some more to see if it helps. </p>