Extra class Package::ConditionalFormat and use it in worksheet.rb. it is draft yet.
add class ConditionalFormat and its subclasses to handle conditional_formatting.

  typecode to State/Strategy
  conditional_format.rb is added, that includes ConditionalFormat and its subclasses such as CellIsFormat, AboveAverageFormat, Top10Format, TextOrWithFormat, TimePeriodFormat, BlanksOrErrorsFormat, ColorScaleFormat, DataBarFormat and ExpressionFormat.
move row_col_notation method to Utility module.
worksheet.rb : Extract method range_param_start_cell_for_conditional_formatting, row_col_param_for_conditional_formatting, param_for_conditional_formatting and range_start_cell_for_conditional_formatting
worksheet.rb : use range_param_start_cell_for_conditional_formatting in conditional_formatting.
move check_dimensions method from Worksheet to Utility.
move convert_date_time method from Worksheet to Utility.
move methods from Worksheet to ConditionalFormat.