RF: remove IncorrectTypeException and rename reg._save_fn.
DOC: describe creating of a new data type
DOC: added description of creationg a new operation
DOC: reference docs. for operations and data types
DOC: rewritten documentation to data types
DOC: adopted comments to data types
DOC: rewritten of operation's documentation
DOC: remove datatypes adding convention
DOC: written the documentation to setting widget