Simplify methods for updating data structures according to the format

There are now two methods that cope with the format upgrade: Config.update_format and Data.update_format. Both receive two arguments: file_format and current_format. The former is the value found in the project (ZIP) file. The latter is always equal to the value of the FORMAT variable defined in format.py and can therefore be eliminated. Files data.py and config.py would then need to import the variable FORMAT.