title: Jxl cell alignment
body: <p>i want to know how can i set an alignment for cell in jxl library , it's default alignment is bottom and when i set alignment for <code>WritableCellFormat</code> it throws me this Exception :</p>
  
  <pre><code>jxl.write.biff.JxlWriteException: Attempt to modify a referenced format
  </code></pre>
  
  <p>i set alignment this way : </p>
  
  <pre><code>WritableFont tahomaboldpt = new WritableFont(WritableFont.TAHOMA, 10,WritableFont.BOLD);
  tahomaBold = new WritableCellFormat(tahomaboldpt);
  tahomaBold.setAlignment(Alignment.CENTRE);
  </code></pre>
  
  <p>this throws me Exception in third line.Thanks in Advance</p>