title: Openpyxl setting number format
body: <p>Could please someone show an example of applying the number format to the cell. For example, I need scientific format, form would be like '2.45E+05' but I couldn't figure a way how to do that in openpyxl.</p>
  
  <p>I tried in several ways but they are all reporting errors when saving the workbook.</p>
  
  <p>for example:</p>
  
  <pre><code>    import openpyxl as oxl
  
      wb = oxl.Workbook()
      ws = wb.create_sheet(title='testSheet')
      _cell = ws.cell('A1')
      _cell.style.number_format = '0.00E+00'
  </code></pre>
  
  <p>or this (here I'm trying to use some of the predefined number formats, I have also seen there is engineering format in builtins but don't know how to access it:</p>
  
  <pre><code>    nf = oxl.style.NumberFormat.FORMAT_NUMBER_00
      _cell.style.number_format = nf
  </code></pre>
  
  <p>In both cases I get the same error:</p>
  
  <pre><code>C:\Python27\openpyxl\cell.pyc in is_date(self)
      408         """
      409         return (self.has_style
  --&gt; 410                 and self.style.number_format.is_date_format()
      411                 and isinstance(self._value, NUMERIC_TYPES))
  
  AttributeError: 'str' object has no attribute 'is_date_format'
  </code></pre>
  
  <p>I have seen this question: <a href="https://stackoverflow.com/questions/8440284/setting-styles-in-openpyxl">Setting styles in Openpyxl</a>
  but it doesn't help because I don't have to change any other formatting settings.</p>