- no longer using file suffix to choose the format
- added 'mformat' argument to 'write_xxx()' functions
- file suffix is now set by 'Message.to_file()' per default
- '.msg' suffix is always used for writing
- 'Message.to_file()' will set the file suffix if the given file_path has none
- added 'mformat' argument to 'Message.to_file()' for choosing the file format
- '.txt' and '.yaml' suffixes are only supported for reading