Writes the header to file.

def write_case_data(self, file):
        """ Writes the header to file.
        """
        case_sheet = self.book.add_sheet("Case")
        case_sheet.write(0, 0, "Name")
        case_sheet.write(0, 1, self.case.name)
        case_sheet.write(1, 0, "base_mva")
        case_sheet.write(1, 1, self.case.base_mva)