ebnf.EBNFCompiler and parsers.Grammar now raise errors when parsing rules can never be reached from the root parser

- syntaxtree.py: transformation_factory decorator for processing table functions; further type annotations

- syntaxtree.py: reversed parameter order in transformer-functions: node is now always the first paramter again.

- parser.py: bug: pos values not initialized in nodes in history record, in case root parser did not match

- ebnf.py: fehlerhafte Typenannotation in der automatisch generierten Compiler-Klasse korrigiert. (Rückgabetyp hängt ganz vom Compiler ab und sollte deshalb nicht mit "str" vorbesetzt sein).

- can use operators "+" (Sequence) and "|" (Alternative) for constructing parsers in Python; better testing suppoer

- example MLW extended by adding rudimentary testcases

syntaxtree, class Node: XML-output now puts tags and content of the innermost node on one line for better readability