fix in-TeX metadata parsing
Refactoring around FlagSet

Error messages on unused flags will probably be improved.
fix review diff output
change some flags regarding test papers

Remove 'with-problem-scores' and 'with-mark-limits' flags in favor of
'no-problem-scores' and 'no-mark-limits'.
Change appearance of postwords in test papers

Make use of $test$duration metadata value.
refactor driver.py
Refactor and fix flags.py

FlagSet is now FlagContainer (since it is not a set).