def to_xml_string(str = '')
      str << ('<conditionalFormatting sqref="' << sqref << '">')
      str << rules.collect{ |rule| rule.to_xml_string }.join(' ')
      str << '</conditionalFormatting>'
    end
Serializes the conditional formatting element
@example Conditional Formatting XML looks like:
<conditionalFormatting sqref="E3:E9">
<cfRule type="cellIs" dxfId="0" priority="1" operator="greaterThan">
<formula>0.5</formula>
</cfRule>
</conditionalFormatting>
@param [String] str
@return [String]