title: change number_format of whole column in openpyxl
body: <p>In Python, using Openpyxl, is there a way of changing the number format of a whole column? </p>
  
  <p>Currently, I'm only able to change this one cell at a time:</p>
  
  <pre><code>wb = openpyxl.load_workbook(xlsxFile)
  ws = wb['Tasks']
  ws.cell(9, 10).number_format = u'#,##0.00€'
  </code></pre>
  
  <p>For confitional formatting, there's this simple solution using a range: </p>
  
  <pre><code>ws.conditional_formatting.add('I2:%s' % row, openpyxl.formatting.rule.CellIsRule(operator='lessThan', formula=['0'], font=redFont))
  </code></pre>
  
  <p>I populate datas in this xlsx file from a python script so one cell at a time is doable but I want to change <code>number_format</code> and conditional formatting for multiple columns and hundreds of rows...</p>