add python exception for ReadParser with threads=0

### fixed issue #86:
- add python exception when ReadParser with threads=0;
- add  test_with_zero_threads() function in tests/test_read_parsers.py;
should the test fail if the exception is not raised? if so, add an 'assert 0, "should fail"' at the end of the 'try' block (right after the rparser statement).